"The surfaces z=f(x,y) and z=g(x,y) intersect in a curve C. Find an expression, in terms of the derivatives of f and g, for a vector tangent to C at a point P on C where x=a and y=b."

Here is what I did.

(1) let F(x,y) = f(x,y) - g(x,y) = 0

(2) I found the gradient vector for F

(3) I found a vector perpendicular to the gradient vector for F, this vector, which I called Txy must give the direction where F(x,y) doesn't change values, and this vector must be in the direction of the curve C.

(4) I found the partial derivative of f(x,y) in the direction of Txy, I believe that this gives the rate of change of z along a direction (Txy) in the xy plane.

(5) I said that the answer was...

(i component of Txy)i +(j component of Txy)j + (|Txy| * partial derivative found in (4) ) k

The book gives the same answer, except it does not include |Txy| in the k component, instead giving just the value of the partial derivative. I couldn't find any reason why this length should be 1, and I think that it needs to be there. Any help?