=lim x(x^2+100-x^2)/[(x^2+100)^1/2 - x]
=100 * lim x/[-x(1+100/x^2)^1/2 - x]
=100 * lim 1/[-(1+100/x^2)^1/2 - 1]
=100 * 1/[-(1+0)^1/2 - 1]
=-100 * 1/2
=-50