Hi i wonder what the wrong is in my calculation is :

deside the tangent and the normal to the curve f(x) = ln(x + sqrt(1 + x^2) in the pont (0,0).

f´(x)= 1/(sqrt(x^2 + 1))

f´(0) = 1/(sqrt(0^2 + 1)) = 1

y = kx + m, y = 0, x = 0, k = 1

m = y -kx = 1 - 1 * 0 = 0

but I must have done wrong somewere, for the right answer is tangen, y= x, and the normal, y = -x.

help someone :/