Question

Find S11 for 3 + 1 + (-1) + (-3) +… A. -77 B. -75 C. -73 D. -71

Answer 1

$r=1-3=-2 \\ \\ a_{1}=3 \\ \\ n=11 \\ \\ S_{11}= \frac{2*3+(11-1)*(-2)}{2} *11= \frac{6-2*10}{2}*11=-7*7=-77$

\\\ Ben