Question

find the sum of the first 15 numbers in a sequnce that has a1 = 6 and d = 4.
a. 62
b.42
c.158
d.510

Answer 1

Given Common Difference : d = 4
and the first term : \(a_1=6\)

Answer 2

hence this is in arithmetic progression ....

you must know the formula of sum of n terms of an arithmetic progression :

\(\Huge{S_n=\frac{n}{2}({2a+(n-1)d})}\)

Answer 3

hence
\(\Huge{S_{15}=\frac{15}{2}(2(6)+(15-1)4)}\)

\(\Huge{S_{15}=\frac{15}{2}(12+56)}\)

\(\Huge{S_{15}=15*34}\)

\(\Huge{S_{15}=510}\)

Answer 4

thanks.