解ï¼ï¼1ï¼
积å0 pai/4 xsinxdx
解ï¼åæ¯=-积å0 pai/4 xdcosx
=积åpai/4 0 xdcosx
=(xcosx/pai/4 0-积åcosxdx)
=(0-pai/4cospai/4-sinx/pai/4 0)
=(-pai/4x2^1/2/2-(0-sinpai/4)
=(-2^1/2pai/8-(-2^1/2/2))
=-2^1/2pai/8+2^1/2/2
(2)积å1 e xlnxdx
æ¢å
æ³
令t=lnx
x:[1,e]
t:[ln1,lne]=[0,1]
x=e^t
dx=e^tdt
åæ¯=积å0 1 e^txtxe^tdt
=积å0 1 te^2tdt
=1/2积å0 1te^2td2t
=1/2积å0 1tde^2t
=1/2(te^2t-积åe^2tdt)
=1/2(te^2t-1/2积åe^2td2t)
=1/2(te^2t-1/2e^2t)
=1/4e^2+1/4
çï¼çæ¡æ¯1/4e^2+1/4ãã
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