Question

qual a soma dos 20 primeiro termo da p.A
( 2,7,12 )

Answer 1

a1 = 2
r = 7 - 2 = 5

an = a1 + ( n - 1) . r
a20 = 2 + ( 20 - 1) . 5
a20 = 2 + (19) . 5
a20 = 2 + 95
a20 = 97

Sn = (a1 + an) . n
                2
S20 =( 2 + 97 ) . 20
                   2
S20 = ( 99 ) . 20
                 2
S20 = 1.980
             2
S20 = 990

Answer 2

$\textit{Resolu\c{c}\~ao}\Rightarrow \left\{\begin{matrix} P.A (2,7,12)\\ r=7-2 =5\\ an=a1+(n-1)r\\ an=2+(19)5 an=2+95 =97\\ \\ S20= \frac{(a1+an)n}{2} \\ S20= \frac{(2+97)20}{2} \\ S20= 990\\ & \boxed{\boxed{Soma=990}}& \end{matrix}\right.$