Question

calcular a soma dos 20 primeiros termos da P.A. (-2,-4,-6, ...)

Answer 1

Sn=n( a1+an)
___---------
_____2

Sn= 20(-2+(-40)
___-----------------
_______2

Sn= 20.(-2-40)
___---------------
______2

Sn= 10.(-42)
Sn= -420


r= a2-a1
r= -4-(-2)
r= -4+2
r= -2


a20= A1+19r
a20= -2+19.(-2)
a20= -2-38
a20= -40