y=x*根号下x^2-1
y'=根号下(x^2-1)+x*1/2*(x^2-1)^(-1/2) *2x=根号下(x^2-1)+x*(x^2-1)^(-1/2)
y=ln[根号下(x^2+4)-x]
y'=1/[根号下(x^2+4)-x] *[1/2*(x^2+4)^(-1/2)-1]
=[1/2**(x^2+4)^(-1/2)-1]/[根号下(x^2+4)-x]