Tìm x , biết : a) ( x + 7 )^2 – x ( x – 3 ) = 12 b) ( 2x + 3 )^2 – 4x^2 =10

`a)(x+7)^2-x(x-3)=12.`

`⇒x^2+2.x.7+7^2-x.x-x.(-3)=12.`

`⇒x^2+14x+49-x^2+3x=12.`

`⇒(x^2-x^2)+(14x+3x)+49=12.`

`⇒17x+49=12.`

`⇒17x=12-49.`

`⇒17x=-37.`

`⇒x=-37/17.`

`b)(2x+3)^2-4x^2=10.`

`⇒(2x)^2+2.2x.3+3^2-4x^2=10.`

`⇒4x^2+2.2x.3+3^2-4x^2=10.`

`⇒4x^2+12x+9-4x^2=10.`

`⇒(4x^2-4x^2)+12x+9=10.`

`⇒12x+9=10.`

`⇒12x=10-9.`

`⇒12x=1.`

`⇒x=1/12.`