=lim (1- 1/(1+x²)) /(k· x^(k-1))
=lim (1+x² - 1) /(k· x^(k-1)) · lim(1/(1+x²) )
=lim (x²) /(k· x^(k-1)) ·1
=lim 1 /(k· x^(k-3))
∴若k≠3,则c=0;
若k=3,则c=1/3