Coordinate Geometry

273 questions

Question 261Question

In the standard (x,y)(x, y) coordinate plane, a circle is defined by the equation x2+y212x+6y+20=0x^2 + y^2 - 12x + 6y + 20 = 0. What is the distance from the center of this circle to the point (9,1)(9, 1)?

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Answer: 5

Answer

The distance from the center of the circle to the point (9,1)(9, 1) is 5.
Completing the square transforms x2+y212x+6y+20=0x^2 + y^2 - 12x + 6y + 20 = 0 into standard form (x6)2+(y+3)2=25(x - 6)^2 + (y + 3)^2 = 25, establishing the center of the circle at (6,3)(6, -3). Applying the coordinate distance formula between (6,3)(6, -3) and (9,1)(9, 1) yields (96)2+(1(3))2=32+42=25=5\sqrt{(9 - 6)^2 + (1 - (-3))^2} = \sqrt{3^2 + 4^2} = \sqrt{25} = 5.

Step-by-Step Solution

1
Group the xx and yy terms and move the constant term to the right side of the equation.
(x212x)+(y2+6y)=20(x^2 - 12x) + (y^2 + 6y) = -20
Grouping terms isolates the variables to prepare for completing the square.
2
Complete the square for both variable expressions.
(x6)2+(y+3)2=20+36+9=25(x - 6)^2 + (y + 3)^2 = -20 + 36 + 9 = 25
Adding (12/2)2=36( -12 / 2 )^2 = 36 and (6/2)2=9( 6 / 2 )^2 = 9 to both sides converts the equation to standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2.
3
Determine the coordinates of the center (h,k)(h, k).
Center is (6,3)(6, -3)
In the standard circle equation (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2, the center is given by (h,k)(h, k).
4
Calculate the distance between the center (6,3)(6, -3) and the point (9,1)(9, 1) using the distance formula.
d=(96)2+(1(3))2=32+42=25=5d = \sqrt{(9 - 6)^2 + (1 - (-3))^2} = \sqrt{3^2 + 4^2} = \sqrt{25} = 5
The distance formula d=(x2x1)2+(y2y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} measures the straight-line distance between two points.

Key Concept

Equations of Circles and Coordinate Distance
Question 262Question

In the standard (x,y)(x, y) coordinate plane, a circle is defined by the equation x2+y28x+10y+k=0x^2 + y^2 - 8x + 10y + k = 0, where kk is a constant. If the circle is tangent to the yy-axis, what is the value of kk?

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Answer: 25

Answer

The value of kk is 25.
Completing the square for x2+y28x+10y+k=0x^2 + y^2 - 8x + 10y + k = 0 gives (x4)2+(y+5)2=41k(x - 4)^2 + (y + 5)^2 = 41 - k. The center of the circle is (4,5)(4, -5). Because the circle is tangent to the yy-axis, its radius is equal to the absolute value of the center's xx-coordinate, which is 4=4|4| = 4. Therefore, r2=16r^2 = 16. Setting 41k=1641 - k = 16 gives k=25k = 25.

Step-by-Step Solution

1
Rewrite the general equation of the circle by completing the square for both xx and yy.
(x28x+16)+(y2+10y+25)+k1625=0    (x4)2+(y+5)2=41k(x^2 - 8x + 16) + (y^2 + 10y + 25) + k - 16 - 25 = 0 \implies (x - 4)^2 + (y + 5)^2 = 41 - k
Converting to standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 reveals the center (h,k)(h, k) and radius squared r2r^2.
2
Identify the center of the circle and determine the radius using the tangency condition.
Center is (4,5)(4, -5). Since the circle is tangent to the yy-axis (the line x=0x = 0), the radius rr equals the horizontal distance from the center to the yy-axis: r=4=4r = |4| = 4.
Tangency to a vertical line means the distance from the center to that line is equal to the radius.
3
Equate the radius squared from step 1 to r2r^2 from step 2 and solve for kk.
41k=42    41k=16    k=2541 - k = 4^2 \implies 41 - k = 16 \implies k = 25
The right side of the standard circle equation represents r2r^2.

Key Concept

Standard form of a circle equation and conditions for tangency to coordinate axes
Question 263Question

A circle in the standard (x,y)(x, y) coordinate plane is represented by the equation x2+y2+4x6y12=0x^2 + y^2 + 4x - 6y - 12 = 0. What is the distance between the two points where this circle intersects the xx-axis?

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Answer: 8

Answer

The distance between the two points where the circle intersects the xx-axis is 8 units.
Setting y=0y = 0 gives the quadratic equation x2+4x12=0x^2 + 4x - 12 = 0. Factoring yields (x+6)(x2)=0(x + 6)(x - 2) = 0, giving solutions x=6x = -6 and x=2x = 2. The distance between these two xx-intercepts is 2(6)=82 - (-6) = 8 units.

Step-by-Step Solution

1
Substitute y=0y = 0 into the circle equation.
x2+4x12=0x^2 + 4x - 12 = 0
Points on the xx-axis have a yy-coordinate of 0.
2
Solve the quadratic equation for xx by factoring.
x=6x = -6 and x=2x = 2
Factoring (x+6)(x2)=0(x + 6)(x - 2) = 0 yields the xx-coordinates of the intersection points.
3
Find the distance between (6,0)(-6, 0) and (2,0)(2, 0).
2(6)=82 - (-6) = 8
The horizontal distance between two points (x1,0)(x_1, 0) and (x2,0)(x_2, 0) is x2x1|x_2 - x_1|.

Key Concept

Finding xx-intercepts of a circle given in general form
Question 264Question

A circle in the standard (x,y)(x, y) coordinate plane is described by the equation x2+y26x+10y+18=0x^2 + y^2 - 6x + 10y + 18 = 0. A second circle is concentric with the first circle (meaning they share the exact same center) but has an area that is 44 times the area of the first circle. Which of the following is an equation of the second circle?

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Answer: (x3)2+(y+5)2=64(x - 3)^2 + (y + 5)^2 = 64

Answer

The equation of the second circle is (x3)2+(y+5)2=64(x - 3)^2 + (y + 5)^2 = 64.
Completing the square on x2+y26x+10y+18=0x^2 + y^2 - 6x + 10y + 18 = 0 yields (x3)2+(y+5)2=16(x - 3)^2 + (y + 5)^2 = 16, identifying the center as (3,5)(3, -5) and r12=16r_1^2 = 16. Concentric circles share the center (3,5)(3, -5). Quadrupling the area means the new area is 4×16π=64π4 \times 16\pi = 64\pi, so r22=64r_2^2 = 64. Plugging the center and new r2r^2 into the standard equation gives (x3)2+(y+5)2=64(x - 3)^2 + (y + 5)^2 = 64.

Step-by-Step Solution

1
Convert the given circle equation to standard form by completing the square for both xx and yy.
Rearranging terms: (x26x)+(y2+10y)=18(x^2 - 6x) + (y^2 + 10y) = -18.
Adding (6/2)2=9( -6/2 )^2 = 9 and (10/2)2=25( 10/2 )^2 = 25 to both sides yields:
(x3)2+(y+5)2=18+9+25=16(x - 3)^2 + (y + 5)^2 = -18 + 9 + 25 = 16.
Standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 reveals the center (h,k)(h, k) and squared radius r2r^2.
2
Identify the center and radius of the original circle.
Center = (3,5)(3, -5) and radius squared r12=16r_1^2 = 16 (so r1=4r_1 = 4).
Concentric circles share the exact same center (h,k)=(3,5)(h, k) = (3, -5).
3
Determine the radius squared of the second circle based on the area constraint.
The area of a circle is A=πr2A = \pi r^2. Since A2=4A1A_2 = 4 A_1, we have πr22=4(πr12)    r22=4r12=4(16)=64\pi r_2^2 = 4 (\pi r_1^2) \implies r_2^2 = 4 r_1^2 = 4(16) = 64.
Quadrupling the area quadruples the value of r2r^2.
4
Write the standard form equation for the second circle using center (3,5)(3, -5) and r22=64r_2^2 = 64.
(x3)2+(y+5)2=64(x - 3)^2 + (y + 5)^2 = 64.
Substituting h=3h = 3, k=5k = -5, and r22=64r_2^2 = 64 into (xh)2+(yk)2=r22(x - h)^2 + (y - k)^2 = r_2^2.

Key Concept

Standard form of a circle equation (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 and area relationship A=πr2A = \pi r^2
Question 265Question

In the standard (x,y)(x, y) coordinate plane, a circle is tangent to the horizontal line y=7y = 7. The equation of the circle is x2+y210x+4y+c=0x^2 + y^2 - 10x + 4y + c = 0, where cc is a constant. What is the value of cc?

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Answer: -52

Answer

The value of cc is 52-52.
Completing the square gives (x5)2+(y+2)2=29c(x - 5)^2 + (y + 2)^2 = 29 - c. The center is (5,2)(5, -2). The distance from (5,2)(5, -2) to the horizontal line y=7y = 7 is 7(2)=9|7 - (-2)| = 9, which is the radius rr. Squaring the radius gives r2=81r^2 = 81. Setting 29c=8129 - c = 81 gives c=52c = -52.

Step-by-Step Solution

1
Complete the square for xx and yy terms.
(x5)2+(y+2)2=29c(x - 5)^2 + (y + 2)^2 = 29 - c
Writing the equation in standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 reveals the center (h,k)(h, k) and the expression for r2r^2.
2
Calculate the radius using the distance from the center to the tangent line.
Radius r=9r = 9
The center is (5,2)(5, -2) and the tangent line is y=7y = 7. The distance between y=2y = -2 and y=7y = 7 is 7(2)=9|7 - (-2)| = 9.
3
Set r2=81r^2 = 81 and solve for cc.
c = -52
Equating the radius squared expression 29c29 - c to 8181 yields 29c=81    c=5229 - c = 81 \implies c = -52.

Key Concept

Converting circle equations to standard form and using distance to tangent lines to determine radius.
Question 266Question

A circle in the standard (x,y)(x, y) coordinate plane is represented by the equation x2+y2+10x6y+9=0x^2 + y^2 + 10x - 6y + 9 = 0. If this circle is translated 44 units to the right and 22 units down, which of the following is the equation of the translated circle?

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Answer: (x+1)2+(y1)2=25(x + 1)^2 + (y - 1)^2 = 25

Answer

(x+1)2+(y1)2=25(x + 1)^2 + (y - 1)^2 = 25
Completing the square on x2+y2+10x6y+9=0x^2 + y^2 + 10x - 6y + 9 = 0 yields (x+5)2+(y3)2=25(x + 5)^2 + (y - 3)^2 = 25, identifying the original center as (5,3)(-5, 3) and r2=25r^2 = 25. Shifting the center 44 units right and 22 units down moves (5,3)(-5, 3) to (5+4,32)=(1,1)(-5 + 4, 3 - 2) = (-1, 1). Substituting the updated center (1,1)(-1, 1) and r2=25r^2 = 25 into the standard equation of a circle (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 gives (x+1)2+(y1)2=25(x + 1)^2 + (y - 1)^2 = 25.

Step-by-Step Solution

1
Group terms and complete the square for both xx and yy variables in the original equation.
(x+5)2+(y3)2=25(x + 5)^2 + (y - 3)^2 = 25
Rearrange x2+y2+10x6y+9=0x^2 + y^2 + 10x - 6y + 9 = 0 as (x2+10x)+(y26y)=9(x^2 + 10x) + (y^2 - 6y) = -9. Add (10/2)2=25(10/2)^2 = 25 and (6/2)2=9(-6/2)^2 = 9 to both sides: (x+5)2+(y3)2=9+25+9=25(x + 5)^2 + (y - 3)^2 = -9 + 25 + 9 = 25.
2
Determine the center and radius of the original circle.
Original center is (5,3)(-5, 3) and r2=25r^2 = 25 (radius r=5r = 5).
Comparing (x+5)2+(y3)2=25(x + 5)^2 + (y - 3)^2 = 25 to (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 gives h=5h = -5, k=3k = 3, and r2=25r^2 = 25.
3
Calculate the coordinates of the new center after the horizontal and vertical translation.
New center is (1,1)(-1, 1).
Translating 44 units to the right adds 44 to the xx-coordinate: 5+4=1-5 + 4 = -1. Translating 22 units down subtracts 22 from the yy-coordinate: 32=13 - 2 = 1.
4
Substitute the new center (1,1)(-1, 1) and unchanged radius squared value r2=25r^2 = 25 into standard form.
(x+1)2+(y1)2=25(x + 1)^2 + (y - 1)^2 = 25
Using (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 with h=1h = -1 and k=1k = 1 gives (x(1))2+(y1)2=25(x - (-1))^2 + (y - 1)^2 = 25, which simplifies to (x+1)2+(y1)2=25(x + 1)^2 + (y - 1)^2 = 25.

Key Concept

Converting circle equations to standard form by completing the square and performing translations in the coordinate plane.
Question 267Question

In the standard (x,y)(x, y) coordinate plane, the equation of a circle is given by 3x2+3y212x+24y15=03x^2 + 3y^2 - 12x + 24y - 15 = 0. What is the radius of this circle?

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Answer: 5

Answer

The radius of the circle is 5 units.
Dividing the equation 3x2+3y212x+24y15=03x^2 + 3y^2 - 12x + 24y - 15 = 0 by 3 gives x2+y24x+8y5=0x^2 + y^2 - 4x + 8y - 5 = 0. Completing the square for both variables results in (x2)2+(y+4)2=25(x - 2)^2 + (y + 4)^2 = 25. Because r2=25r^2 = 25, taking the square root gives the radius r=5r = 5.

Step-by-Step Solution

1
Divide the entire equation by 3.
x2+y24x+8y5=0x^2 + y^2 - 4x + 8y - 5 = 0
The standard form of a circle equation (xh)2+(yk)2=r2(x-h)^2 + (y-k)^2 = r^2 requires the coefficients of x2x^2 and y2y^2 to be 1.
2
Group terms and move the constant to the right side of the equation.
(x24x)+(y2+8y)=5(x^2 - 4x) + (y^2 + 8y) = 5
Isolating quadratic terms makes completing the square straightforward.
3
Complete the square for both variables by adding (42)2=4(\frac{-4}{2})^2 = 4 and (82)2=16(\frac{8}{2})^2 = 16 to both sides.
(x24x+4)+(y2+8y+16)=5+4+16(x^2 - 4x + 4) + (y^2 + 8y + 16) = 5 + 4 + 16
Adding these values turns the expressions into perfect square trinomials while preserving equality.
4
Express in standard form and solve for the radius rr.
(x2)2+(y+4)2=25    r2=25    r=5(x - 2)^2 + (y + 4)^2 = 25 \implies r^2 = 25 \implies r = 5
The radius is the principal square root of r2r^2.

Key Concept

Converting general form of a circle equation to standard form using completing the square.
Question 268Question

An ellipse in the standard (x,y)(x, y) coordinate plane is defined by the equation

(x2)29+(y+5)216=1\frac{(x-2)^2}{9} + \frac{(y+5)^2}{16} = 1

What are the coordinates of the center of this ellipse?

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Answer: (2,5)(2, -5)

Answer

The center of the ellipse is (2,5)(2, -5).
The standard form of an ellipse equation is (xh)2a2+(yk)2b2=1\frac{(x-h)^2}{a^2} + \frac{(y-k)^2}{b^2} = 1, where the center of the ellipse is at the coordinate point (h,k)(h, k). Comparing the given equation (x2)29+(y+5)216=1\frac{(x-2)^2}{9} + \frac{(y+5)^2}{16} = 1 to the standard form reveals that h=2h = 2 and k=5k = -5. Therefore, the center coordinates are (2,5)(2, -5).

Step-by-Step Solution

1
Recall the standard form equation of an ellipse with horizontal/vertical axes.
The standard form is (xh)2a2+(yk)2b2=1\frac{(x-h)^2}{a^2} + \frac{(y-k)^2}{b^2} = 1, where (h,k)(h, k) represents the coordinates of the center.
This establishes the framework to extract the center coordinate values.
2
Compare the terms in the given equation to the standard form.
Matching (xh)2(x-h)^2 with (x2)2(x-2)^2 gives h=2h = 2. Matching (yk)2(y-k)^2 with (y+5)2=(y(5))2(y+5)^2 = (y-(-5))^2 gives k=5k = -5.
Comparing terms identifies the offsets hh and kk that determine the center.
3
Write the center coordinate pair (h,k)(h, k).
The center is (2,5)(2, -5).
Combining the values of hh and kk yields the final coordinates.

Key Concept

Identifying the center of an ellipse from its standard form equation
Estimated Time:45s
Question 269Question

In the standard (x,y)(x, y) coordinate plane, line l1l_1 passes through the points (2,5)(-2, 5) and (4,1)(4, 1). Line l2l_2 is perpendicular to line l1l_1. What is the slope of line l2l_2?

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Answer: 32\frac{3}{2}

Answer

The slope of the perpendicular line is 32\frac{3}{2}.
To find the slope of a line perpendicular to a given line, first calculate the slope of the original line, l1l_1, using the slope formula m=y2y1x2x1m = \frac{y_2 - y_1}{x_2 - x_1}. Substituting the points (2,5)(-2, 5) and (4,1)(4, 1) into the formula gives m1=154(2)=46=23m_1 = \frac{1 - 5}{4 - (-2)} = \frac{-4}{6} = -\frac{2}{3}. The slope of a perpendicular line, l2l_2, is the negative reciprocal of the slope of l1l_1. The negative reciprocal of 23-\frac{2}{3} is 32\frac{3}{2}.

Step-by-Step Solution

1
Calculate the slope of the line l1l_1 passing through the points (2,5)(-2, 5) and (4,1)(4, 1) using the slope formula.
m1=154(2)=46=23m_1 = \frac{1 - 5}{4 - (-2)} = \frac{-4}{6} = -\frac{2}{3}
The slope mm of a line passing through (x1,y1)(x_1, y_1) and (x2,y2)(x_2, y_2) is given by m=y2y1x2x1m = \frac{y_2 - y_1}{x_2 - x_1}.
2
Find the slope of line l2l_2, which is perpendicular to l1l_1, by taking the negative reciprocal of m1m_1.
m2=1m1=123=32m_2 = -\frac{1}{m_1} = -\frac{1}{-\frac{2}{3}} = \frac{3}{2}
The product of the slopes of two perpendicular lines is 1-1, so the slope of a perpendicular line is the negative reciprocal of the original slope.

Key Concept

Slope of perpendicular lines and finding slope from two points.
Estimated Time:1m 0s
Question 270Question

In the standard (x,y)(x, y) coordinate plane, a triangle has vertices at A(0,1)A(0, 1), B(2,3)B(2, 3), and C(8,11)C(8, 11). If point MM is the midpoint of side ABAB and point NN is the midpoint of side ACAC, what is the length of the line segment MNMN?

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Answer: 5

Answer

The length of the line segment MNMN is 55.
The length of the line segment MNMN is 55. Calculating the coordinates of the midpoint of ABAB, we get M(1,2)M(1, 2). For ACAC, the midpoint is N(4,6)N(4, 6). Applying the distance formula between MM and NN yields (41)2+(62)2=9+16=5\sqrt{(4-1)^2 + (6-2)^2} = \sqrt{9+16} = 5. Alternatively, by the Midsegment Theorem, the segment connecting the midpoints of two sides of a triangle is half the length of the third side. The length of the third side BCBC is (82)2+(113)2=36+64=10\sqrt{(8-2)^2 + (11-3)^2} = \sqrt{36 + 64} = 10, so the length of MNMN is 102=5\frac{10}{2} = 5.

Step-by-Step Solution

1
Find the coordinates of MM, the midpoint of side ABAB with endpoints A(0,1)A(0, 1) and B(2,3)B(2, 3).
M(1,2)M(1, 2)
The midpoint formula is M=(x1+x22,y1+y22)M = \left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right).
2
Find the coordinates of NN, the midpoint of side ACAC with endpoints A(0,1)A(0, 1) and C(8,11)C(8, 11).
N(4,6)N(4, 6)
Applying the midpoint formula gives (0+82,1+112)=(4,6)\left(\frac{0 + 8}{2}, \frac{1 + 11}{2}\right) = (4, 6).
3
Calculate the distance between M(1,2)M(1, 2) and N(4,6)N(4, 6) using the distance formula.
55
The distance formula is d=(x2x1)2+(y2y1)2d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}. Here, d=(41)2+(62)2=32+42=25=5d = \sqrt{(4-1)^2 + (6-2)^2} = \sqrt{3^2 + 4^2} = \sqrt{25} = 5.

Key Concept

Midpoint and Distance Formulas
Question 271Question

A vertex of a polygon in the standard (x,y)(x, y) coordinate plane undergoes two transformations: it is first reflected across the line y=xy = x, and then it is translated 44 units to the right. If the coordinates of the final image of the vertex are (1,5)(1, 5), what were the coordinates of the vertex before the transformations?

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Answer: (5,3)(5, -3)

Answer

The coordinates of the vertex before the transformations were (5,3)(5, -3).
To find the pre-image of the vertex, we apply the inverse transformations in reverse order. First, we undo the translation of 44 units to the right by translating the image point (1,5)(1, 5) by 44 units to the left, resulting in (3,5)(-3, 5). Second, we undo the reflection across the line y=xy = x by swapping the coordinates of (3,5)(-3, 5), which yields the original point (5,3)(5, -3).

Step-by-Step Solution

1
Identify the inverse of the second transformation (translation of 44 units to the right).
The inverse is a translation of 44 units to the left, which subtracts 44 from the xx-coordinate: (14,5)=(3,5)(1 - 4, 5) = (-3, 5).
To find the pre-image, we must undo the transformations in reverse order, starting with the last transformation applied.
2
Identify the inverse of the first transformation (reflection across the line y=xy = x).
The inverse of a reflection across y=xy = x is itself, which swaps the xx- and yy-coordinates: (3,5)(5,3)(-3, 5) \rightarrow (5, -3).
Undoing the first transformation on the intermediate coordinates gives the original pre-image coordinates.

Key Concept

Working backward from a final image using inverse transformations in reverse order.

Alternative Method

We can write the composite transformation as an algebraic rule. A reflection across y=xy = x maps (x,y)(y,x)(x, y) \to (y, x). A translation 44 units to the right maps (y,x)(y+4,x)(y, x) \to (y + 4, x). Setting the final coordinates equal to (1,5)(1, 5), we get y+4=1y + 4 = 1 (which means y=3y = -3) and x=5x = 5. Thus, the original coordinates were (5,3)(5, -3).
Estimated Time:1m 15s
Question 272Question

The equation x2+y212x+8y+3=0x^2 + y^2 - 12x + 8y + 3 = 0 defines a circle in the standard (x,y)(x, y) coordinate plane. What is the radius of this circle?

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Answer: 7

Answer

The radius of the circle is 7.
Completing the square on the given equation yields (x6)2+(y+4)2=49(x - 6)^2 + (y + 4)^2 = 49. By comparing this to the standard circle equation (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2, we find r2=49r^2 = 49. Taking the square root of 4949 gives a radius of 77.

Step-by-Step Solution

1
Group the xx and yy terms and move the constant to the right-hand side of the equation.
x212x+y2+8y=3x^2 - 12x + y^2 + 8y = -3
This groups terms containing the same variables together to prepare for completing the square.
2
Add the square of half of the linear coefficients to both sides to complete the square for both xx and yy.
(x212x+36)+(y2+8y+16)=3+36+16(x^2 - 12x + 36) + (y^2 + 8y + 16) = -3 + 36 + 16
Adding (122)2=36(\frac{-12}{2})^2 = 36 and (82)2=16(\frac{8}{2})^2 = 16 to both sides forms perfect square trinomials on the left side while maintaining equality.
3
Factor the trinomials into squared binomials and combine the constants on the right side.
(x6)2+(y+4)2=49(x - 6)^2 + (y + 4)^2 = 49
This rewrites the equation in the standard circle equation form.
4
Extract the radius from the standard form equation (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2.
r=7r = 7
Since the constant on the right side corresponds to r2r^2, taking the square root of 4949 gives the radius of the circle.

Key Concept

Converting a circle's equation from general form to standard form by completing the square to determine its properties.
Question 273Question

In the standard (x,y)(x, y) coordinate plane, a circle is defined by the equation x2+y2+14x6y+9=0x^2 + y^2 + 14x - 6y + 9 = 0. What is the area, in square units, of the region enclosed by the circle?

Show answer & explanation

Answer: 49π49\pi

Answer

The area of the region enclosed by the circle is 49π49\pi square units.
To find the area of the circle, we rewrite the equation x2+y2+14x6y+9=0x^2 + y^2 + 14x - 6y + 9 = 0 in standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2. Regrouping gives (x2+14x)+(y26y)=9(x^2 + 14x) + (y^2 - 6y) = -9. Completing the square for xx requires adding (142)2=49(\frac{14}{2})^2 = 49, and for yy requires adding (62)2=9(\frac{-6}{2})^2 = 9. Adding both to the right side gives 9+49+9=49-9 + 49 + 9 = 49. Thus, the standard equation is (x+7)2+(y3)2=49(x + 7)^2 + (y - 3)^2 = 49, yielding r2=49r^2 = 49. The area enclosed by the circle is πr2=49π\pi r^2 = 49\pi.

Step-by-Step Solution

1
Group terms and move the constant term to the right side of the equation.
(x2+14x)+(y26y)=9(x^2 + 14x) + (y^2 - 6y) = -9
Grouping xx-terms and yy-terms prepares the equation for completing the square.
2
Complete the square for both xx and yy expressions by adding (142)2=49(\frac{14}{2})^2 = 49 and (62)2=9(\frac{-6}{2})^2 = 9 to both sides.
(x2+14x+49)+(y26y+9)=9+49+9(x^2 + 14x + 49) + (y^2 - 6y + 9) = -9 + 49 + 9
Adding these values turns the quadratic expressions into perfect square trinomials while keeping the equation balanced.
3
Write in standard circle equation form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 to identify r2r^2.
(x+7)2+(y3)2=49(x + 7)^2 + (y - 3)^2 = 49, so r2=49r^2 = 49
The standard form directly gives the radius squared, r2=49r^2 = 49.
4
Calculate the area of the circle using the formula A=πr2A = \pi r^2.
A=π(49)=49πA = \pi (49) = 49\pi
Substituting r2=49r^2 = 49 into the area formula gives the final enclosed area.

Key Concept

Converting general circle equation x2+y2+Dx+Ey+F=0x^2 + y^2 + Dx + Ey + F = 0 to standard form (xh)2+(yk)2=r2(x - h)^2 + (y - k)^2 = r^2 by completing the square.
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