Bearing in mind that an absolute value expression is in effect a
piece-wise function with two cases, thus
Let
. The gradient of
at the point (1, 0, 0) is the normal vector to the surface, which is also orthogonal to the tangent plane at this point.
So the tangent plane has equation

Compute the gradient:

Evaluate the gradient at the given point:

Then the equation of the tangent plane is

y=45x+200
y is the total cost, x is the number of months, and 200 is the start.
Pretty sure its number 2, 8m do you need the whole process?