(1)设f(t)=∫0tex2dx,求∫01t2f(t)dt. (2)设f(x)=∫0xecostdt,求∫0πf(x)cosxdx.

admin2019-09-04  25

问题 (1)设f(t)=∫0tex2dx,求∫01t2f(t)dt.
(2)设f(x)=∫0xecostdt,求∫0πf(x)cosxdx.

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答案(1)∫01t2f(t)dt=[*]∫01f(t)d(t3)=[*]f(t) |01-[*]∫01t3et2dt, 因为f(1)=0,所以 ∫01t2f(t)dt=[*]∫02t3er2dt=[*]∫01t2et2d(t2) =[*]∫01xexdx=[*]sin3xdx=[*], (2)∫0πf(x)cosxdx=∫0πf(x)d(sinx)=f(x)sinx|0π-∫0πf’(x)sinxdx =-∫0πf’(x)sinxdx=-∫0πecosxsinxdx=∫0πecosxd(cosx) =ecosx0π=e-1-e.

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