Question

determine a soma dos 15 primeiros termos da p.a. (11,14,17,20,...)​

Answer 1

$\Large\boxed{\begin{array}{l}\sf a_n=a_1+(n-1)\cdot r\\\sf a_{15}=11+(15-1)\cdot3\\\sf a_{15}=11+14\cdot3\\\sf a_{15}=11+42\\\sf a_{15}=53\end{array}}$

$\Large\boxed{\begin{array}{l}\sf S_n=\dfrac{(a_1+a_n)\cdot n}{2}\\\\\sf S_{15}=\dfrac{(11+53)\cdot 15}{2}\\\\\sf S_{15}=\dfrac{64\cdot 15}{2}\\\\\sf S_{15}=32\cdot15\\\sf S_{15}=480~\checkmark\end{array}}$