Question

find the sum of the first m terms of an ap whose nth term is (5-6n). hence also find the sum of its first 20 terms !! pls solve it quickly !! thank u !!

Answer 1

Given:
t(n)= 5 - 6n

Sum of the first 20 terms

= n/2 [a + l]

= 20/2 [ t(1) + t(20)]

= 10 [ (5-6) + (5-120)]

= 10 [ -1 -115]

= 10 × [ - 116] = - 1160

I hope this answer helps you

Answer 2

The sum of the first 20 terms of the given AP is -1160.

The general term = 5 - 6n

First term = 5 - 6(1) = -1

mth term = 5 - 6(m)

Sum (S) = m/2 (first term + mth term)

              = m/2 (4 - 6m)

Putting m = 20, S = 10 x (-116)

                = -1160