3. The equation (+ + y) = (r - y) defines a curve which intersects the r-axis in three points. The gradients of the curve at these three points are. in some order, as follows. (N.B. 'undefined' means that the tangent line is vertical at that point.) (a) 0. 0 and undefined. (b) 1, 0 and (c) 1, - and (d) 1, 0 and undefined (e) 0, 0 and 3. (f) -3, 0 and