a/2=b/3
∴2b=3a
∴:(5a-2b)/(a²-4b²)·(a-2b)
=(5a-2b)/(a+2b)(a-2b)×(a-2b)
=(5a-2b)/(a+2b)
=(5a-3a)/(a+3a)
=2a/4a
=1/2