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Find the equation of the set of all points the sum of whose distances from the points (3, 0) and (9, 0) is 12.

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Let the coordinates of the variable point be (x,y)

 Then according to the question \sqrt{\left ( x-3 \right )^{2}+y^{2}}=12-\sqrt{\left ( x-9 \right )^{2}+y^{2}}

x2-6x+9+y2=144+x^{2}-18x+81+y^{2}-24\sqrt{\left (x-9 \right )^{2}+y^{2}}

\left ( x-18 \right )=-2\sqrt{\left (x-9 \right )^{2}+y^{2}}

Again squaring both sides, we get 

x2-36x+324=4(x2-18x+81+y2)

3x2+4y2-36x=0  which is an ellipse.

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