∫ f(x) dx = (sinx)/x
f(x) = d/dx (sinx)/x = (xcosx - sinx)/x²
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∫ xf'(x) dx
= ∫ x df(x)
= ∫ x df(x)
= xf(x) - ∫ f(x) dx
= x • (xcosx - sinx)/x² - (sinx)/x + C
= (xcosx - sinx)/x - (sinx)/x + C
= cosx - 2(sinx)/x + C
积分xf'(x)dx＝积分（xdf(x)）=xf(x)－积分(f(x)dx)=xf(x)－sinx/x+C
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