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Another Point In A Square (Posted on 2004-06-11) Difficulty: 3 of 5
There is a point M inside a square ABCD such that angle MAB is 60 and angle MCD is 15. Find angle MBC.

See The Solution Submitted by Brian Smith    
Rating: 3.2500 (4 votes)

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Solution solution | Comment 4 of 16 |

Let point M have coordinates (x,y) and the square have unit sides, with D at the origin and B at (1,1) and A at (0,1).

Since MAB is 60 degrees, MAD is 30 degrees.

So x = (1-y)tan 30

Since angle MCD is 15 degrees,

y = (1-x)tan 15

Substituting this expression for y in the first equation,

x = (1 - tan15 + x tan 15) tan 30

This solves to

x = (1-tan15)tan30/(1-(tan15)(tan30))

This latter comes out to x = .5, so M is on the vertical centerline of the square.

By symmetry then, angle MBC is the same as angle MAD, or 30 degrees.


Edited on June 11, 2004, 9:37 am
  Posted by Charlie on 2004-06-11 09:35:30

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