(-x-5)(-x+5)=x^2-5x+5×-25=x^2-25
(-2x+1)2=-4x+2
(5x-1)(x+2)=5x^2+10x-x-2=5x^2+9x-2
(x^2-25)(-4x+2)=-4x^3+2x^2+100x-50| :2 => -2x^3+x^2+50x-25=x^2(-2x+1)+25(2x-1)=(x^2+25)(2x-1)
(x^2+25)(2x-1)=0 => x^2+25=0 sau 2x-1=0
x^2=-25 sau 2x=1
x=+/-5 sau x=1/2
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