(3-6b)/(4b^2-1)
=3(1-2b)/(2b+1)(2b-1)
=-3/(2b+1)
(2x+4)/(2+x)
=2(x+2)/(x+2)
=2
a+2b=0
a/b=-2
(a^2+2ab-b^2)/(2a^2+ab+b^2)
分子分母同时除以b^2
=(a/b^2+2a/b-1)/(2a/b^2+a/b+1)
=(4-4-1)/(8-2+1)
=-1/7