x^4+6x^2+9=(x^4 -9) +6x^2+18
x^4+6x^2+9=x^4-9+6x^2+18
x^4+6x^2+9=x^4+6x+9
чтд
(x^2+3)^2=(x^2-3)(x^2+3)+6(x^2+3)
x^4+6*x^2+9-(x^2-3)*(x^2+3)-6*(x^2+3)=0
x^4+6*x^2+9-x^4+9-6*(x^2+3)=0
6x^2+18-6*(x^2+3)=0
6x^2+18-(6x^2+18)=0
6x^2+18-6x^2-18=0
18-18=0
0=0
Тождество доказано!
