1、[arctan(1/x)]'=[1/(1+1/x^2)]*(-1/x^2)=-1/(1+x^2)
[arctan(1/x)]"=[-1/(1+x^2)]'=2x/[(1+x^2)]^2
[xln(根号下x)]'=ln(根号下x)+x[1/ln(根号下x)][-1/2(根号下x)]=ln(根号下x)-1/2
[xln(根号下x)]"=[ln(根号下x)-1/2]'=-1/2x
y"=2x/[(1+x^2)]^2-1/2x
2、(lnx/x)dx=lnxd(lnx)=0.5(lnx)^2
3、xcosxdx=xd(sinx)=xsinx|（0到π/2）-sinxdx=π/2 sin(π/2)-0sin0+cosx|（0到π/2）=π/2-[0-1]=π/2+1