由 a^2 - 3b^2 = 2ab,得
a^2 - 2ab - 3b^2 = 0
(a - 3b) * (a + b) = 0,因为a>0,b>0,所以
a=3b,代入表达式,得
(a-2b)/(a+2b) = (3b-2b)/(3b+2b) = 1/5