若f在[0,a]上连续可微,且f(0)=0,则 ∫0a|f(x)f’(x)|dx≤∫0a[f’(x)]2dx.

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问题 若f在[0,a]上连续可微,且f(0)=0,则
    ∫0a|f(x)f’(x)|dx≤0a[f’(x)]2dx.

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答案因为 ∫0a|f(x)f’(x)|dx=∫0a|f(x)|d|f(x)|=[*] 又f(0)=0,f(a)=∫0af’(x)dx,所以∫0a|f(x)f’(x)|dx=[*](∫0af’(x)dx)2. 由施瓦兹不等式可知, (∫0af’(x)dx)2=(∫0af’(x)·ldx)2≤(∫0a(f’(x))2dx)(∫0adx)≤a∫0a(f’(x))2dx. 因此,∫0a|f(x)f’(x)|dx≤[*][f’(x)]2dx.

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