Prove that the diameter is the longest chord in a circle.

An equilateral triangle $ABC$ is inscribed in a circle . $P$ is a point on the arc $BC$. Prove that $PA=PB+PC$ 

Draw a $△ABC$, whose $BC=6 cm, CA=5.5 cm$ and $AB=4.5 cm$ and draw an incircle of $△ABC$