a/b=2a=2b所以a²=4b²ab=2b²原式=(4b²-2b²+3b²)/(4b²+2b²+6b²)=5b²/12b²=5/12
a²-ab+3b²/a²+ab+6b²上下同除以b²=(a/b)²-a/b+3 / (a/b)²+a/b+6=4-2+3 / 4+2+6=5/12
