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 A Cute Triangle (Posted on 2006-07-21)
Given any triangle, through each vertex draw the external angle bisector at that vertex. Show that the new triangle that has as its vertices the three pairwise intersection points of these is always an acute triangle (all three angles strictly less than 90 degrees).

Extra Credit: Extended to meet the new triangle, the internal angle bisectors of the given triangle are what with respect to the new triangle?

 See The Solution Submitted by Richard Rating: 3.0000 (1 votes)

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 Solution | Comment 1 of 4
`Let the original triangle be ABC and the newtriangle be A*B*C* where X* lies on the internalbisector of angle X. The internal and externalbisectors of an angle of ABC are perpendicular.A*, B*, and C* are the excenters of ABC. ABC isthe orthic triangle of A*B*C*. The internal anglebisectors of ABC are the altitudes of A*B*C*. LetI be the incenter of ABC ( it is also theorthocenter of A*B*C*). Let`
`    m(<A) = 2x    m(<B) = 2y          0 < x,y,z < 90    m(<C) = 2z`
`IBA*C is a cyclic quadrilateral.`
`    m(<BA*C) = 180 - m(<BIC)               = 180 - ( m(<BIA*) + m(<CIA*) )             = 180 - ( ( m(IAB) + m(IBA) ) +                     ( ( m(IAC) + m(ICA) ) )             = 180 - ( x+y+z ) - x             = 90 - x < 90`
`A similar argument holds for m(<CB*A) andm(<AC*B). `
` `

 Posted by Bractals on 2006-07-21 09:47:37
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