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You might like to make a tessellation with copies of $\Delta ABC$ and three triangles drawn on the sides of $\Delta ABC$ and prove your conjecture from the tessellation. This uses only elementary geometry. An alternative method is to use the Cosine Rule.
Alternatively you can use vectors or complex numbers.
Napoleon's Theorem
Age 14 to 18
Challenge Level 
You might like to make a tessellation with copies of $\Delta ABC$ and three triangles drawn on the sides of $\Delta ABC$ and prove your conjecture from the tessellation. This uses only elementary geometry. An alternative method is to use the Cosine Rule.
Alternatively you can use vectors or complex numbers.
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