a)(x-3)^3-(x-3)(x^2+3x+9)+9(x+1)^2=15
<=>x^3-9x^2+27x-27-(x^3-3^3)+9(x^2+2x+1)=15
<=>x^3-9x^2+27x-27-x^3+27+9x^2+18x+9=15
<=>(x^3-x^3)+(-9x^2+9x^2)+(27x+18x)-(27-27)=15-9
<=>45x=6
<=>x=2/15
vậy x=2/15
b)x(x-5)(x+5)-(x+2)(x^2-2x+4)=3
<=>x(x^2-25)-(x^3+8)=3
<=>x^3-25x-x^3-8=3
<=>-25x=3+8
<=>-25x=11
<=>x=-11/25
vậy x=-11/25