(1)
å
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sin³x=sinxsin²x=(1/2)sinx(1-cos2x)=(1/2)(sinx-sinxcos2s)
=(1/2)[sinx-(1/2)(sin3x-sinx)]
= (3/4)sinx-(1/4)sin3x
æ以å
æ±åºä¸å®ç§¯å
=(3/4)â«xsinxdx-(1/4)â«xsin3xdx
= -(3/4)â«xdcosx+(1/12)â«xdcos3x
=(1/12)[xcos3x-(1/3)sin3x]-(3/4)[xcosx-sinx]
=(1/12)xcos3x-(3/4)xcosx-(1/3)sin3x+(3/4)sinx+C
ç¶å带å
¥æ±åºï¼
å积å=(1/12)xcos3x-(3/4)xcosx-(1/3)sin3x+(3/4)sinx |ï¼0---ãÏ)
=2Ï/3
(2)
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â«(0->Ï) xf(sinx)dx=(Ï/2) â«(0->Ï) f(sinx)dx
æ以ï¼å积å=â«(0->Ï) xsin³xdx=(Ï/2) â«(0->Ï) sin³xdx=2Ï/3
亲æµæ 误
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