Question

In an Ap the third term is 21 and the ninth term is 39.find the sum of the first twelve terms.

Answer 1

Let the first term and common difference be a and d respectively
a3=21
a+2d=21----------------------(1)
a9=39
a+8d=39-------------------(2)
Eq(1)-eq(2)
-6d=-18
d=3
Put d in eq(1)
a+6=21
a=15
Now S15=15/2(2a+14d)
=15/2(30+42)
=15(15+21)
=15x36
=540

Answer 2

.............................................srry