Question

qual a doma dos 20 primeiros termos da P.A (7, 10, 13...)?​

Answer 1

$r=a_{2} -a_{1} =10-7\Longrightarrow r=3 \\ \\ a_{20} =a_{1} +19r=7+19\cdotp 3\Longrightarrow a_{20} =64 \\ \\ \therefore S_{20} =\frac{( a_{1} +a_{20}) \cdotp 20}{2} =( 7+64) \cdotp 10 \\ \\ \Longrightarrow \boxed{\boxed{S_{20} =710}}$