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设g(x) = ∫<a,x> f(t)dt, 则g'(x) = f(x), g"(x) = f'(x).
g(x)在[a,b]二阶连续可导, 且g(a) = 0, g'(a) = f(a) = 0.
由带Lagrange余项的Taylor展开, 存在c∈(a,b)使
g(b) = g(a)+g'(a)(b-a)+g"(c)(b-a)²/2 = f'(c)(b-a)²/2.
即有| ∫<a,b> f(t)dt| = |g(b)| = |f'(c)|·(b-a)²/2 ≤ max{|f'(x)|}·(b-a)²/2.
g(x)在[a,b]二阶连续可导, 且g(a) = 0, g'(a) = f(a) = 0.
由带Lagrange余项的Taylor展开, 存在c∈(a,b)使
g(b) = g(a)+g'(a)(b-a)+g"(c)(b-a)²/2 = f'(c)(b-a)²/2.
即有| ∫<a,b> f(t)dt| = |g(b)| = |f'(c)|·(b-a)²/2 ≤ max{|f'(x)|}·(b-a)²/2.
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