计算：（3x/x-1-x/x+1）·x²-1/x；（a/a-2-a/a+2）·a²-4/a.

[3x/(x-1) -x/(x+1)]· (x²-1)/x
=3x/(x-1) · (x²-1)/x - x/(x+1)· (x²-1)/x
=3(x+1) - (x-1)
=3x+3-x-1
=2x+2
.
[a/(a-2) -a/(a+2)]· (a²-4)/a
=a/(a-2) · (a²-4)/a - a/(a+2) · (a²-4)/a
=a+2 -(a-2)
=a+2-a+2
=4