Question

a1= 4,2
d= - 3
S15-?
Найти S15.

Answer 1

$a_{1}=4,2\\\\d=-3\\\\a_{15}=a_{1}+14d=4,2+14*(-3)=4,2-42=-37,8\\\\S_{15}=\frac{a_{1}+a_{15}}{2}*15=\frac{4,2-37,8}{2} *15=\frac{-33,6}{2}*15=-16,8*15=-252$