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Given the function f (x) = Log1 / 2 (- ax & # 178; + 2x + 3), if f (x) is an increasing function on [2,4], find the value range of real number a
F (x) is an increasing function on [2,4]
Then: y = - ax & # 178; + 2x + 3 is a decreasing function on [2,4], and Y (min) > 0, that is: Y (4) > 0;
(1) When a = 0, y = 2x + 3, which is obviously not satisfied, is discarded;
(2) A0, get: a
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