Question

calcule a soma dos múltiplos positivos de 9, menores que 100.

Answer 1

M (9)=9 18 27 36 45 54 63 72 81 90 99 depois e só somar tudo.

Answer 2

$P.A.=(9,\cdots,99)\\\\a_n=a_1+r(n-1),\quad p/\ n\in\mathbb{N}\ \ e\ \ n\geq2\\\\99=9+9(n-1)\\\\99=9+9n-9\\\\99=9n\\\\n=\dfrac{99}{9}\\\\n=11\\\\\\S_n=\dfrac{n\cdot(a_1+a_n)}{2}\\\\\\S_{11}=\dfrac{11(9+99)}{2}\\\\\\S_{11}=\dfrac{11(108)}{2}\\\\\\S_{11}=11(54)\\\\\boxed{S_{11}=594}$