[（a²+b²）+2b（a-b）-（a-b）²]÷4b
=(a²+b²+2ab-2b²-a²+2ab-b²)÷4b
=(4ab-2b²)÷4b
=2b(2a-b)÷4b
=(2a-b)/2
=5/2 (2分之5)