Question

Dê a soma dos termos da P.A (12,5,-2,...-30)

Answer 1

-30=12+ (n-1),-7
-30=12+(-7n+7)
-30=12-7n+7
-30-12-7=-7n (.-1)
30+12+7=7n
49=7n
n=49/7
n=7

Sn =12+ (-30).7
        __________
                 2
Sn= 12-210 
       _______
             2 
Sn=-198/2 = -99