Question

a) a soma dos 10 primeiros termos da PA (2, 5, …);

b) a soma dos 15 primeiros termos da PA (– 1, – 7, …);

c) a soma dos 20 primeiros termos da PA (0,5; 0,75, …). ​

Answer 1

Resposta:

Explicação passo-a-passo:

$a)\\S_{n}=\dfrac{(a_{1}+a_{n})\cdot\;n}{2}\\[0,1cm]S_{10}=\dfrac{(2+ 3)\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{5\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{50}{2}\\[0,1cm]S_{10}=\boxed{25}$

$b)\\S_{n}=\dfrac{(a_{1}+a_{n})\cdot\;n}{2}\\[0,1cm]S_{10}=\dfrac{(-1+ (-6))\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{-7\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{-70}{2}\\[0,1cm]S_{10}=\boxed{-35}$

$c)\\S_{n}=\dfrac{(a_{1}+a_{n})\cdot\;n}{2}\\[0,1cm]S_{10}=\dfrac{(0,5+ 0,25)\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{0,75\cdot\;10}{2}\\[0,1cm]S_{10}=\dfrac{7,5}{2}\\[0,1cm]S_{10}=\boxed{3,75}$

Espero ter ajudado.