Question

In an A.P., if the 12th term is -13 and the sum of its four terms is 24, find the sum of its 1st ten terms.

Answer 1

a=9 ; d=-2 ; n= 10.

sum of first 10 terms =

$\frac{n}{2} (2a + (n - 1)d)$

= 5 ((2×9)+(9×−2))

= 5(18 - 18) = 5 × 0 = 0=5(18−18)=5×0=0

therefore, sum of first 10 terms of AP = 0.