Question

calcule a soma dos termos da p.a (3 , 7 , 11 , ..., 99 )

Answer 1

r=7-3
r=4

an=a1+(n-1)r
99=3+(n-1)4
96=4n-4
100=4n
n=25

Sn=(a1+an)n/2
s25=(3+99)25/2
S25=102.25/2
S25=1275