Question

3)calcule a soma dos quinze primeiros termos da P.A (-45,-41,-37,-33...)

Answer 1

a₁₅= -45+(15-1).(-4)
a₁₅= -45+(-56)
a₁₅= -101

S₁₅=(a₁+a₁₅).n ÷2

S₁₅=(-45-101).15 ÷2

S₁₅=(-146).15 ÷2

S₁₅= -2190 ÷2

S₁₅= -1095