Question

calcule a soma dos 50 primeiros termos da p.a (11,13,15,...)

Answer 1

$a_{1} =11$

$r=13-11=2$

$a_{n} =a_{1} +(n-1).r$

$a_{50} =a_{1} +49.r=11+49.2=11+98=109$

$S_{n} = \dfrac{(a_{1} +a_{n}).n}{2}$

$S_{50} = \dfrac{(a_{1} +a_{50}).50}{2}$

$S_{50} = \dfrac{(11 +109).50}{2}$

$S_{50} = \dfrac{(120).50}{2}$

$S_{50} = 60.50$

$S_{50} = 3000$

Answer 2

A50= 11+49.2==> 11+98==> 109

S50= (11+109).50 ==> 120.50
                 2                           2

S50 = 3.000