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Find Points, Maximise Ratio (Posted on 2015-06-05) Difficulty: 3 of 5
Given a plane X, a point M in the plane and a point N not in the plane.

Find all points R in X such that the ratio (NM + MR)/NR is the maximum.

No Solution Yet Submitted by K Sengupta    
Rating: 4.0000 (1 votes)

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solution | Comment 1 of 6
Let a be then angle MRN
b the angle MNR
and c the angle NMR
then from law of sines we have

then (MN+MR)/NR = (MN/NR)+(MR/NR)
= [sin(a)+sin(b)]/sin(c)
this is maximized when a=b thus we need

Now rotate the plane in question so it coincides with the xy plane.  And flip if needed so M is above the plane.

Now let the points be at the following coordinates:
N (a,b,0)
M (p,q,r)
R (x,y,0)
then we need

this is the same as saying that R needs to be equidistant from N as a point at (p,q,0)
(p,q,0) is simply the point M projected onto the chosen plane.

Thus the locus of R is simply a circle on the plane with center at N and radius equal to the distance from N to the projection of M onto the plane.

  Posted by Daniel on 2015-06-05 13:02:15
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