Question:

If the sum of the first four terms of an A.P. is 6 and the sum of its first six terms is 4, then the sum of its first twelve terms is

-22

-24

-20

-26

Step-by-Step Solution

Key Concept: $S_4=4a+6d=6\Rightarrow2a+3d=3$. $S_6=6a+15d=4$. Solve: $d=-5/6$, $a=11/4$.

$S_{12}=-22$.

Correct Answer: 1