Question

Find S25 for (-7) + (-2) + 3 + 8 +...

A.
1,320

B.
1,325

C.
825

D.
820

Answer 1

$a_1=-7 \\ a_2=-2 \\ \\ d=a_2-a_1=-2-(-7)=5 \\ \\ \\ S_{25}= \cfrac{2(-7)+(25-1)5}{2}\cdot25 = \cfrac{-14+120}{2}\cdot25 =\cfrac{106}{2}\cdot25 =1325$

Your answer is B. 1325