Question

calcule a soma dos 15 primeiros termos da pa ( -45-41-37-33) ?

Answer 1

$S_{n} = \frac{(a_{1}+ a_{n} ).n }{2} \\ \\ a_{1} =-45 \\ \\ r= a_{2} - a_{1} =-41-(-45)=-41+45=4 \\ \\ a_{n} = a_{1} +(n-1).r \\ \\ a_{15} =(-45)+(15-1).4 \\ \\ a_{15} =-45+14.4=-45+56=11 \\ \\ S_{15} = \frac{(-45+11).4}{2} = \frac{-34.4}{2} = \frac{-136}{2} =-68$