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#1 2014-07-08 03:43:16

rptamin
Member
Registered: 2014-07-02
Posts: 3

Proof

Let the incircle of triangle ABC be tangent to sides

,
, and
at
,
, and
, respectively. Prove that triangle DEF is acute.

[Edited latex by admin]

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#2 2014-07-09 01:55:56

rptamin
Member
Registered: 2014-07-02
Posts: 3

Re: Proof

Please help,I need a few tips, I am not getting anywhere with it.

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#3 2014-07-25 08:22:33

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,337

Re: Proof

hi rptamin

Let O be the centre of the circle.

The incentre always lies inside the triangle => DOE < 180

By angle property => DFE < 90. 

Ditto for the other two angles.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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