The plane of equation x + y = 1 intersects the cone of equation z = 4 - sqrt(x^2 + y^2) in a parabola. Verify that the point of coordinates ( 3/7 , 4/7 , 23/7 ) is on that parabola and find the equation of the line tangent to the parabola at the given point.
Hint: Parameterize the parabola by x = t, y =?, z =?