Đặt biến phụ dạng đẳng cấp: a) 3(-x^2 + 2x + 3)^4 - 26x^2 (-x^2 + 2x + 3)^2 - 9x^4; b) (x^2 - x + 2)^4 - 3x^2 (x^2 - x + 2)^2 + 2x^4; c) -6(-x^2 - x +1)^4 + x^2(-x^2 - x +1)^2 + 5x^4

a) 3(-x^2 + 2x + 3)^4 - 26x^2 (-x^2 + 2x + 3)^2 - 9x^4
b) (x^2 - x + 2)^4 - 3x^2 (x^2 - x + 2)^2 + 2x^4
c) -6(-x^2 - x +1)^4 + x^2(-x^2 - x +1)^2 + 5x^4
d) (x^2 - x - 1)^4 + 7x^2(x^2 - x - 1)^2 + 12x^4
e) 10(x^2 - 2x + 3)^4 - 9x^2(x^2 - 2x + 3)^2 - x^4