Question

find the sum of first 15terms of ap whose nth term is an=n+3​

Answer 1

Answer:

n+3=1+3=4

A=4,

2+3=5

d=1

a15=a+14d

        4+14*1=18

sum of 15th term is= n/2(a+l)

                       15/2(4+18)=165........................

Explanation:

hope it is correct..

Answer 2

an = n+3

first term = 1+3 => a1 = 4

second term = 2+3 => a2 = 5

third term = 3+3 => a3 = 6

so d = 1

Sn = n/2{2a+(n-1)d}

=> S15 = 15/2{ 2× 4 + 14 × 1}

=> S15 = 15/2 × { 8+14}

=> S15 = 15/2 × 22

=> S15 = 15×11

=> S15 = 165

so the sum of first 15th term of an AP is 165

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