已知a=x^2-7x-2,b=-2x^2+4x-1,计算：1.a+b 2.b+a .a-b 4.b-a

a+b
=x²-7x-2+(-2x²+4x-1)
=x²-7x-2-2x²+4x-1
=-x²-3x-3
b+a
=-2x²+4x-1+x²-7x-2
=-x²-3x-3
a-b
=x²-7x-2-(-2x²+4x-1)
=x²-7x-2+2x²-4x+1
=3x²-11x-1
b-a
=-2x²+4x-1-(x²-7x-2)
=-2x²+4x-1-x²+7x+2
=-3x²+11x+1