=(a+2b)(a-2b)/(a-2b) +4b²/(a-2b)-（a的平方-4b的平方分之4a的平方b)
=(a²-4b²+4b²)/(a-2b) -4a²b/(a+2b)(a-2b)
=a²/(a-2b)-4a²b/(a+2b)(a-2b)
=[a²(a+2b)-4a²b]/(a+2b)(a-2b)
=a²(a-2b)/(a+2b)(a-2b)
=a²/(a+2b)