∵a=3√ab-2b,
∴√ab=1/3(a+2b),
(√a-√b)/(√a+√b)
=(√a-√b)^2/(a-b)
=(a+b-2√ab)/(a-b)
=(3a+3b-2a-4b)/3(a-b)
=(a-b)/3(a-b)
=1/3