`a)`
`7x(2x-100)-x(7x+10)=200-x`
`<=> 14x^2-700x-7x^2-10x=200-x`
`<=> 14x^2-7x^2-700x-10x+x-200=0`
`<=> 7x^2-709x-200=0`
`<=> x^2-709/7x-200/7=0`
`<=> x^2 - 2 . x . 709/14 + (709/14)^2 - 200/7 - (709/14)^2=0`
`<=> (x-709/14)^2 - 508281/196 = 0`
`<=> (x-709/14)^2 = 508281/196`
`<=>` \(\left[ \begin{array}{l}x-\dfrac{709}{14}=\dfrac{\sqrt{508281}}{14}\\x-\dfrac{709}{14}=-\dfrac{\sqrt{508281}}{14}\end{array} \right.\) `<=>` \(\left[ \begin{array}{l}x=\dfrac{709+\sqrt{508281}}{14}\\x=\dfrac{709-\sqrt{508281}}{14}\end{array} \right.\)
Vậy `x=(709+-sqrt(508281))/14`
`b) `
`4x^2-20x+10=4x(x-10)`
`<=> 4x^2-20x+10=4x^2-40x`
`<=> 4x^2-4x^2-20x+40x=-10`
`<=> 20x=-10`
`<=> x=-1/2`
Vậy `x=-1/2`
`c)`
`(x+10)10x-100=x(10x+20)`
`<=> 10x^2+100x-100=10x^2+20x`
`<=> 10x^2-10x^2+100x-20x=100`
`<=> 80x=100`
`<=> x=5/4`
Vậy `x=5/4`
