Question

the first term of an AP is -3 and the 10th term is 15 find S10​

Answer 1

First term of AP, a= -3

and, the last term of AP, a₁₀=15

and, number of terms, n=10

now, S₁₀= n/2(a+a₁₀)

            =10/2(-3+15)

            =5(12)

            =60

∴sum of 10 terms of the AP is 60.

Hope it helps..

Answer 2

S₁₀ = 60

Given :

The first term of an AP is - 3 and the 10th term is 15

To find :

The value of S₁₀

Formula :

For a arithmetic progression if first term is a and nth term is The nth term of an AP is aₙ then

Sum of first n terms

= Sₙ

$\displaystyle \sf = \frac{n}{2} \bigg[a + a_n \bigg]$

Solution :

Step 1 of 2 :

Write down first term and 10th term

First term = a = - 3

10th term = a₁₀ = 15

Step 2 of 2 :

Find the value of S₁₀

S₁₀

= Sum of first 10 terms

$\displaystyle \sf = \frac{10}{2} \bigg[a + a_{10} \bigg]$

$\displaystyle \sf = 5 \times ( - 3 + 15)$

$\displaystyle \sf = 5 \times 12$

$\displaystyle \sf = 60$

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