Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

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

Let the incircle of triangle ABC be tangent to sides

, , and at , , and , respectively. Prove that triangle DEF is acute.[Edited latex by admin]

Offline

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

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

Offline

**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,801

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

Offline