I found some questions in which i have doubts. Please answer in detail along with diagram or graph where needed.

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3. Let pi = (-1,0), pa = (1,0), and consider the set of all points f = (r, y) such that the sum of the distance between F and pi and the distance between f and pz is a constant k > 2. e.g. IT - pll+ f - pill = k (a) Describe the set of points forming the curve F. (b) For k = 4 sketch the curve. 5. Consider f(x,y) = vry at the point P(2, 8) moving toward the point Q(5, 4). (a) Find the gradient of f at P (b) Find the directional derivative of f at P in the direction of