设f(x)在[0,+∞)上可导,f(0)=0,且f(x)的反函数为g(x),若∫0f(x)g(t)dt=∫0f(x)tsin2t/(sint+cost)dt,求f(π/2)。

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问题 设f(x)在[0,+∞)上可导,f(0)=0,且f(x)的反函数为g(x),若∫0f(x)g(t)dt=∫0f(x)tsin2t/(sint+cost)dt,求f(π/2)。

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答案已知等式两边同时对x求导,得g[f(x)]f’(x)=xsin3x/(sinx+cosx),又由g[f(x)]=x,可知当x≠0时,有f’(x)=sin3x/(sinx+cosx),故f(π/2)=f(0)+∫0π/2sin3x/(sinx+cosx)dx=∫0π/2sin3x/(sinx+cosx)dx,又由于∫0π/2sin3x/(sinx+cosx)dx→∫0π/2cos3t/(sint+cost)dt=∫0π/2cos3x/(sinx+cosx)dx,故f(π/2)=∫0π/2sin3x/(sinx+cosx)dx=1/2∫0π/2(sin3x+cos3x)/(sinx+cosx)dx=1/2∫0π/2(sin2x-sinxcosx+cos2x)dx=1/2∫0π/2(1-sinxcosx)dx=1/2(π/2-1/2sin2x∫0π/2)=(π-1)/4。

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