Đặt biến phụ dạng hồi quy: a) x^4 + 6x^3 + 7x^2 + 6x + 1; b) x^4 + 5x^3 - 12x^2 + 5x + 1; c) 6x^4 + 5x^3 - 38x^2 + 5x + 6; d) x^4 - 10x^3 + 26x^2 - 10x + 1; e) x^4 + 7x^3 + 14x^2 + 14x + 4

a) x^4 + 6x^3 + 7x^2 + 6x + 1
b) x^4 + 5x^3 - 12x^2 + 5x + 1
c) 6x^4 + 5x^3 - 38x^2 + 5x + 6
d) x^4 - 10x^3 + 26x^2 - 10x + 1
e) x^4 + 7x^3 + 14x^2 + 14x + 4
f) x^4 - 10x^3 - 15x^2 + 20x + 4
g) 2x^4 - 5x^3 -27x^2 + 25x + 50
h) 3x^4 + 6x^3 - 33x^2 - 24x + 48