Question

Find the sum of the arithmetic progression:

– 26, – 24, – 22, …. to 36 terms

Answer 1

In an A.P if the first term = a, common difference = d, and if there are n terms.

Then, sum of n terms is given by:

Given expression -26, – 24. -22, to 36 terms

First term (a) = -26

Common difference (d) = -24 – (-26)

= -24 + 26 = 2

Sum of n terms, 

for n = 36

S_n = 36/2{2(-26) + (36 – 1)2}

= 18[-52 + 70]

= 18×18

= 324

∴ S_n = 324