Question

Find the sum of first n terms of an AP whose nth term is (5 - 6n). Hence, find the sum of its first 20 terms

Answer 1

Let an be the nth term of the AP.

∴ an = 5 - 6n

Putting n = 1, we get

First term, \(a=a_1=5-6\times1=-1\)

Putting n = 2, we get

\(a_2=5-6\times2=-7\)

Let d be the common difference of the AP.

\(\therefore d=a_2-a_1=-7-(-1)=-7+1=-6\)

Sum of first n term of the AP, Sn