Question 36866
For each arithmetic series, find S of 25"
3+7+11+15

S(25) means to find the sum of the 1st 25 terms.
d is the difference between any term and the previous term:
For your problem d=11-7=4
The Sum formula is S(n)=(n/2)(a(1)+a(n))
and a(n)=a(1)+(n-1)d
Since a(1)=3, a(25)=3+(24)(4)=99
Therefore S(25)=(25/2)(3+99)=25(51)=1275
Cheers,
stan H.