Question

find sum of first 12 terms of an AP whose third term is
18 and sixth term is 9

Answer 1

Answer:

Step-by-step explanation:

an=a+(n-1)d

18=a+2d

9=a+5d

By solving above equations,

d=-3

Substitute 'd' value in any above equation.

U will get 'a' value.

Substitute 'a' value in formula (given in first line)

U will get a12=-9