`a)`
`x(x^2 + x +1) - x^2 ( x+1) - x15`
`=x^3+x^2+x-x^3-x^2-15x`
`=(x^3-x^3)+(x^2-x^2)+(x-15x)`
`=-14x`
``
Tìm x
`a)`
`(x-1)(x+1)-x(x-5)=15`
`\to x^2-1-x^2+5x=15`
`\to (x^2-x^2)+5x-1=15`
`\to 5x=16`
`\to x=16/5`
Vậy `x=16/5`
``
`b)`
`x(5x-10)-5x^2=50`
`\to 5x^2-10x-5x^2=50`
`\to (5x^2-5x^2)-10x=50`
`\to -10x=50`
`\to x=-5`
Vậy `x=-5`