The sum of the first n terms of an arithmetic progresssion is 252. If the first term is -16 and the last term is 72, find the number of terms in the series
-
A.
6 -
B.
7 -
C.
8 -
D.
9
Correct Answer: Option D
Explanation
(S_n = 252, a = -16hspace{1mm}andhspace{1mm}l = 72\S_n = frac{n}{2}(-16+72)\252 = frac{n}{2}(-16+72)\n=frac{504}{56}\n=9)