Find the equation of the set of all points the sum of whose distances from the points (3, 0) and (9, 0) is 12.
Let the coordinates of the variable point be (x,y)
Then according to the question
x2-6x+9+y2=
Again squaring both sides, we get
x2-36x+324=4(x2-18x+81+y2)
3x2+4y2-36x=0 which is an ellipse.