Question

encontre a soma dos 20 primeiros termos da P.A(3,6,9,12...)

Answer 1

$an=a_1+(n-1)*r\\ \\a_{20}=3+(20-1)*3\\ \\a_{20}=58$

Soma

$Sn= \frac{(a_n+a_1)n}{2} \\ \\S_{20}= \frac{(58+3)*20}{2} \\ \~S_{20}=310$