[(a²＋b²)+2b(a－b)－(a－b)²]÷4b=(a²+b²+2ab-2b²-a²+2ab-b²)÷4b
=(4ab-2b²)÷4b
=a-b/2
若2a-b=6 b=2a-6,则
原式=a-a+3=3