将a/b=2即a=2b代入:
(a²-ab+3b²)/(a²+ab+6b²)
=[(2b)²-(2b)b+3b²]/[(2b)²+(2b)b+6b²]
=(4b²-2b²+3b²)/(4b²+2b²+6b²)
=5/12.