Question

1+3+5+...+x=36

Answer 1

x=11

Answer 2

Số số hạng là (x-1):2+1(số)

Tổng là: \(\dfrac{\left(x+1\right)\cdot\left[\dfrac{1}{2}\left(x-1\right)+1\right]}{2}=\dfrac{1}{4}\left(x+1\right)\left(x-1\right)+\dfrac{1}{2}\left(x+1\right)\)

Theo đề, ta có: \(\dfrac{1}{4}\left(x+1\right)\left(x-1\right)+\dfrac{1}{2}\left(x+1\right)=36\)

\(\Leftrightarrow\left(x+1\right)\left(x-1\right)+2\left(x+1\right)=144\)

\(\Leftrightarrow x^2-1+2x+2-144=0\)

=>x²+2x-143=0

=>x=11