Question

The sum of 20 and 28 terms of an A.P. are equal. Find the sum of 48 terms of this A.P.

Answer 1

S₂₀ = S₂₈

`S_20 = 20/2[2a + (20 - 1)d] = S_28 = 28/2[2a + (28 - 1)d]`

`20/2[2a + 19d] = 28/2[2a + 27d]`

10a + 95d = 14a + 189d

−4a = 94d

a = `(-94d)/4`

a = `(-47d)/2`

2a = −47d

`S_48 = 48/2[2a + 47d]`

= 24[−47d + 47d]

= 24 × 0

= 0