Question

19. Find the sum of first 15 terms of 3 + 6 + 9 using the formula​

Answer 1

Answer:

360

Step-by-step explanation:

a=3

d=3

n=15

then by applying formula S15= n/2 (a+(n-1)d)

S15= 15/2 (6+42)

= 15/2(48)

= 360

Answer 2

Answer:

here 1st term is a=3

common difference d =(6-3)=3

sum of 1st 15 terms

=15/2{2.a+(15-1).d

=15/2{2.3+14.3}

=15/2{6+42)

=15/2.48

=15.24

=360