As shown in the figure, △ ABC, ab = 9, AC = 6, ad ⊥ BC at point D, M is any point on ad, then the value of mb2-mc2 is______ ．

In RT △ abd and RT △ ADC, BD2 = ab2-ad2, CD2 = ac2-ad2; in RT △ BDM and RT △ CDM, BM2 = BD2 + MD2 = ab2-ad2 + MD2, MC2 = Cd2 + MD2 = ac2-ad2 + MD2, ｜ mb2-mc2 = - (ac2-ad2 + MD2) + (ab2-ad2 + MD2) = ab2-ac2 = 45

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