Question

find the sum of first 15 terms of a gp 2,4,8​

Answer 1

Answer:

Here a

1

=24,a

2

=21

Common difference d=a

2

−a

1

=21−24=−3

⇒a

n

=a

1

+(n−1)dn

th

term formula

We first find the value of n for which the value of n is zero

The next integer value of that n will be the term of first negative number

⇒a

n

=a

1

+(n−1)d

⇒0=24+(n−1)(−3)

⇒−24=(n−1)−3

⇒8=n−1

⇒n=9

Hence, 10

th

term will be the first negative term of given AP

a

10

=a

1

+(n−1)d=24+9×(−3)=−3