Question

if the 12th term of an ap is -13 and the sum of the first four terms is 24,what is the sum of first 10 terms. ​

Answer 1

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

sum of first 10 terms =

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

$= \frac{10}{2} ((2 \times 9) + (9 \times - 2))$

$= 5(18 - 18) = 5 \times 0 = 0$

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