Question

Find the sum of first 20 terms of an A.P. whose n^th term is given by T_n = (7 - 3n).

(A) 382

(B) -490

(C) 420

(D) -382

Answer 1

The correct option is: (B) -490

Explanation:

We have, T_n = (7 -3n)

First term, T₁ = (7 -3 x 1) = 4

Second term, T₂ = 7 - 3 x 2= 1

Third term, T₃ = 7 -3 x 3 = -2

.'. Series is 4, 1, - 2, ..........

and common difference = -3

Sum of first 20 terms (S₂₀)

= 20/2[2 x 4 + (20 - 1)(-3)] = 10[8 - 57] = -490