11. Find the partial sum S12 of the arithmetic series 26 + 24 +22+ 20+....
+605 The formula is \(S_{n}=\frac{(T_1+T_n)n}{2}\). Note that t_12=(26+2)-(2*12)=28-24=4. Note that t_1=26. So we have \(S_{12}=\frac{(4+26)12}{2}=\frac{30*12}{2}=30*6=\boxed{180}\)
