一,a平方-4分之1- a-2分之1=
=(1-(a+2))/(a^2-4)
=(-1-a)/(a^2-4)
=-(1+a)/(a^2-4)
二,x平方-4x+4分之x平方-4 -x-2分之1
=(x+2)(x-2)/(x-2)^2-1/(x-2)
=(x+2)/(x-2)-1/(x-2)
=(x+2-1)/(x-2)
=(x+1)/(x-2)
x-1分之2+1-x分之x-1
=2/x-1+(-(1-x))/(1-x)
=2/x-1-1
=(2-x+1)/(x-1)
=(3-x)/(x-1)