证明∫f(t)f'(t)dt = 1/2 [f(b)^2 - f(a)^2]

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证明∫f(t)f'(t)dt = 1/2 [f(b)^2 - f(a)^2]

证明∫f(t)f'(t)dt = 1/2 [f(b)^2 - f(a)^2]
证明∫f(t)f'(t)dt = 1/2 [f(b)^2 - f(a)^2]

证明∫f(t)f'(t)dt = 1/2 [f(b)^2 - f(a)^2]
∫(a->b)f(t)f'(t)dt
=∫(a->b)f(t)df(x)
=1/2∫(a->b)2f(t)df(x)
=1/2∫(a->b)d[f(x)^2]
=1/2f(x)^2|(a->b)
=1/2[f(b)^2-f(a)^2]

记p=∫f(t)f'(t)dt
用分部积分法:p=f(t)*f(t)-∫f(t)f'(t)dt =f(t)^2-p=f(b)^2-f(a)^2-p
所以移项,除以2得:p=1/2*[f(b)^2-f(a)^2]

∫f(t)f'(t)dt = ∫f(t)df(t)
=[(1/2)f(t)*f(t)]|带入上下限a,b