Question

Calcule a soma dos quinzes primeiros termos da P.A (-45,-41,-37,-33...)

Answer 1

S15=?
a15= ?
r=4 
a15= a1.14.r
a15= -45.14.4
a15=-45.56
a15=11 

S15= 15(-45+11)/2 
S15=15. -34/2
S15=15. -17
S15= -255