帮忙解答三道因式分解题！

（m+n)^3+2mn(1-m-n)-1
=(m+n)^3-1-2mn(m+n-1)
=(m+n-1)(m^2+mn+n^2)-2mn(m+n-1)
=(m+n-1)(m^2+n^2-mn)

(a^2+a+1)(a^2+a+2)-12
=(a^2+a)^2+3(a^2+a)+2-12
=(a^2+a+5)(a^2+a-2)
=(a^2+a+5)(a+2)(a-1)

12x^4-56x^3+89x^2-56x+12
=(x-2)(12x^3-32x^2+25x-6)
=(x-2)(2x-1)(6x^2-13x+6)
=(x-2)(2x-1)(2x-3)(3x-2)

12x^4-56x^3+89x^2-56x+12
=12(x^4+1)-56x(x²+1)+89x²
=12(x²+1)²-56x(x²+1)+65x²
=[2(x²+1)-5x][6(x²+1)-13x]
=(x-2)(2x-1)(2x-3)(3x-2)

1)利用立方差公式求解
原式=(m+n)^3-1-2mn(m+n-1)
=(m+n-1)(m^2+mn+n^2)-2mn(m+n-1)
=(m+n-1)(m^2+n^2-mn)
2) 把 (a^2+a)看成一个整体
原式=(a^2+a)^2+3(a^2+a)+2-12
=(a^2+a+5)(a^2+a-2)
=(a^2+a+5)(a+2)(a-1)
3)原式=12X^4-56X^3+64X^2+25X^2-56X+12
=4X^2(X-2)(3X-8)+(X-2)(25X-6)
=(X-2)(12X^3-32X^2+25X-6)
=(X-2)(2X-1)(6X^2-13X+6)
=(X-2)(2X-1)(2X-3)(3X-2)