第一题：
6x-10x²+12x²-5x
=(12-10)x²+(6-5)x
=2x²+x
第二题：
(x-y)²+7(x+y)-11(x-y)²-8(x+y)
=(1-11)(x-y)²+(7-8)(x+y)
=-10(x-y)²-(x+y)
=-10(x+y)(x-y)-(x-y)
=(-10x-10y-1)(x-y)
第三题：
2x²-5x+x²+4x-3x²-2
=(2+1-3)x²+(4-5)x-2
=-x-2
=-1/2-2
=-5/2
第四题：
x³-5x²-x³-x²+2x+6x²-1
=(1-1)x³+(-5-1+6)x²+2x-1
=2x-1
=2*2008-1
=4015