Tìm x biết: 5x(3-x)+x.(5+5x)

Cách giải:

$5x\left(3-x\right)+x\left(5+5x\right)=40$

$\begin{array}{l}\iff 5x\left(3-x\right)+5x\left(1+x\right)=40\\ \iff 5x\left(3-x+1+x\right)=40\\ \iff 5x.4=40\\ \iff 20x=40\\ \iff x=40:20\\ \iff x=2\end{array}$

Vậy $x=2$ .