1、原式=[(2x^2 +1)-(x^2-1)+(4-5x)]/(x^2-5x+6)
=(x^2-5x+6)/(x^2-5x+6)
=1
2、原式=[a/(a-b)^2]+[a/(a-b)^2]
=(a+a)/(a-b)^2
=2a/(a-b)^2
3、原式=b/(a-b)^3 -b/(a-b)^3
=(b-b)/(a-b)^3
=0
4、原式=[(x+2)/(x-2)]-[(x-4)/(x-2)]
=[(x+2)-(x-4)]/(x-2)
=6/(x-2)
5、原式=[3a/(a-b)]-[(a+b)/(a-b)]+[(3a-2b)/(a-b)]
=[3a-(a+b)+(3a-2b)]/(a-b)
=(5a-3b)/(a-b)
6、原式=[(x+y)/(x-y+z)]-[(x+z)/(x-y+z)]-[(y+z)/(x-y+z)]
=[(x+y)-(x+z)-(y+z)]/(x-y+z)
=-2z/(x-y+z)