Question

if the sum of seven terms of an ap is 49 and that of its first 17 term is 289 find the sum of first n terms of AP

Answer 1

s7=49

=>7/2(2a+(7-1)d)=49=>7/2(2a+6d)=49=>14a+42d=98=>a+3d=7......(1)

s17=289

Similarly by applying above formula

a+8d=27.............(2)

Subtracting 1 and 2 we get,

d=4

substituting this value of d in 2 we get

a=-5

From these value of a and d we get

-2n-7n to the power2