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y=x^x=e^(xlnx)
å æ¤dy/dx=[e^(xlnx)](xlnx)'=[e^(xlnx)](lnx+1)
=(x^x)(lnx+1)
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f'(x)/f(x)=1/x+1/(x+1)+1/(x+2)+......+1/(x+m)
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f '(x)=f(x)[1/x+1/(x+1)+1/(x+2)+......+1/(x+m)]
=[x(x+1)(x+2)......(x+n)][1/x+1/(x+1)+1/(x+2)+......+1/(x+m)]
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f '(x)=(x+1)(x+2)...(x+n)+x(x+2)(x+3)....(x+n)+x(x+1)(x+3)....(x+n)+.....+x(x+1)(x+2)....(x+n-1)
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f '(0)=n!
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