The 4^{\text{th}} term of an arithmetic progression (A.P.) is 15 and the 9^{\text{th}} ter… — JAMB UTME

The $4^{\text{th}}$ term of an arithmetic progression (A.P.) is $15$ and the $9^{\text{th}}$ term is $35$ . Calculate the sum of the first $10$ terms of the progression.

Answer: 210

Answer

The sum of the first 10 terms of the progression is 210.

Using the

n^{\text{th}}

term formula

T_n = a + (n-1)d

, the equations

a + 3d = 15

and

a + 8d = 35

yield common difference

d = 4

and first term

a = 3

. Substituting these into the sum formula

S_n = \frac{n}{2}[2a + (n-1)d]

for

n = 10

yields

S_{10} = 5[2(3) + 9(4)] = 210

Step-by-Step Solution

Set up equations for the given terms

a + 3d = 15

and

a + 8d = 35

The

n^{\text{th}}

term of an A.P. is given by

T_n = a + (n-1)d

Solve for the common difference

d

d = 4

Subtracting

(a + 3d = 15)

from

(a + 8d = 35)

gives

5d = 20

, so

d = 4

Solve for the first term

a

a = 3

Substituting

d = 4

into

a + 3(4) = 15

yields

a = 15 - 12 = 3

Calculate the sum of the first 10 terms

S_{10} = 210

Applying

S_{10} = \frac{10}{2}[2(3) + 9(4)] = 5(6 + 36) = 210

Key Concept

Finding terms and sums of an Arithmetic Progression using simultaneous linear equations

Estimated Time:1m 30s

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