Question

Dada a PA (5,7,9,...), calcule a soma dos primeiros 100 termos

Answer 1

an = a1+(n+-1)r
a100 = 5 + (100-1)2
a100 = 5 + 99*2
a100 = 5 + 198
a100 = 203

Sn = (a1+an)*n/2
Sn = (5+203)*100/2
Sn = (208)*50
Sn = 10400

Answer 2

an = a1 + (n-1).r

a100 = 5 + 99.2 = 5+198 ==> 203
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Sn = (a1+an).n
              2

S100 = (1+203).100 ==> 204.50 ==> 10200
                  2