Analysis and synthesis to prove inequality If positive numbers a and B satisfy AB = a + B + 3, then the value range of AB is?
Ab-3 = a + b > = 2 radical ab
Let t = radical AB,
T^2-2T-3>=0
T>=3 or T=3,
So, AB > = 9 (if and only if a = b = 3 is equal sign)
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