lim(x+c/x-c)=4,x趋向于无穷，要详细过程。

lim(x+c/x-c)^x=4,
=lim((x-c+2c)/x-c)^x=4
=lim(1+2c/x-c)^x=4,
=e^lim2cx/(x-c)=4
=e^2c=4
2c=ln4
c=ln2