## College Algebra (6th Edition)

$S_1:3=\frac{1(1+5)}{2}=\frac{6}{2}$, thus this is true $S_2:3+4=7=\frac{2(2+5)}{2}=\frac{14}{2}$, thus this is true $S_3:3+4+5=12=\frac{3(3+5)}{2}=\frac{24}{2}$, thus this is true