Question

a soma dos 80 elementos iniciais da pa (10,6,2...) e?

Answer 1

a1= 10
r= 6-10= -4
n=80
An= ?
SN=?


An= a1+(n-1).r
An= 10+(80-1).(-4)
An= 10+79.(-4)
An= 10+(-316)
An= 10-316
An= -306


sn= (a1+An).n/2
SN= (10+{-306}).80/2
SN= (10-306).40
SN= -296.40
SN= -11840


Espero ter ajudado! ;)