(a+b)/ab-（b+c）/bc
=[c(a+b)-a(b+c)]/(abc)
=(ca+bc-ab-ac)/(abc)
=b(c-a)/(abc)
=(c-a)/ac
=1/a-1/c
3x/（x-3)²-x/(3-x)
=[3x+x(x-3)]/(x-3)²
=x²/(x-3)²
1/(x-1)+ 1/(1+x)
=[(x+1)+(x-1)]/[(x-1)(1+x)]
=2x/(x²-1)
c/ab - a/bc
=(c²-a²)/(abc)
a/(a²-1) + (3a+1)/(a²-1) + (2a+3)/(1-a²)
=[a+(3a+1)-(2a+3)]/(a²-1)
=([2a-2)/(a²-1)
=2(a-1)/[(a+1)(a-1)]
=2/(a+1)