Question

Determine a soma dos termos de cada PA

(86,83,80,...35)

Answer 1

35= 86 +(n-1) .-3
35 = 86 -3n +3
3n = 86 +3 - 35
3n = 54 
n = 54/3
n = 18
sn = (86 +35) .18/2
sn = 121 .18/2
sn =  2178/2
sn =1089 

Answer 2

..(86,83,80,...35)

a1=86
an=35
r=-3
n=?

formula (an=a1+(n-1).r)
35=86+(n-1).(-3)
35=86-3n+3
35=89-3n
-3n=35-89
-3n=-54 .(-1)
  n=54/3= 18

formula da soma [ sn=(a1+an)n/2]

sn=(86+35)18/2
sn=121.18/2
sn=2178/2=1089

Espero ter ajudado!!!