x+根号下(x^2+1)
=根号下x²+根号下(x^2+1)
=1/[根号下(x^2+1)-根号下x²]
=1/[根号下(x^2+1)-x]
f(a)=lg(a+根号下a^2+1)=b
f(-a)=lg[根号下(a^2+1)-a]
=lg1/[a+根号下(a^2+1)]
=-lg[a+根号下(a^2+1)]
=-b