3. Phép nhân hai số tự nhiên. Dạng 3. Tìm x

$2.x+3=15$

$2.x=15-3$

$2.x=12$

$x=12:2$

$x=6$

$3.x=28-13$

$x=15:3$

$x=5$

$x-1954=50:5$

$x-1954=10$

$x=10+1954$

$x=1964$

$30.\left(x-60\right)=30$

$x-60=30:30$

$x=1+60$

$x=61$

$x+99:3=55$

$x+33=55$

$x=55-33$

$x=22$

$\left(x-25\right):15=20$

$x-25=20.15$

$x-25=300$

$x=300+25$

$x=325$

$\left(3.x-15\right).7=42$

$3.x-15=42:7$

$3.x-15=6$

$3.x=6+15$

$x.\left(x+1\right)=2+4+6+8+10+\mathrm{...}+2500$

$2+4+6+8+10+\mathrm{...}+2500$

$\left(2500-2\right):2+1=1250$

$2+4+6+8+10+\mathrm{...}+2500$

$\begin{array}{l}=\left[\left(2500+2\right).1250\right]:2\\ =1563750\end{array}$

$x.\left(x+1\right)=1563750=1250.1251$

$x=1250$