We know that the angle of an n sided regular Polygon is equal to \((\frac{2n-4}{n})\) right angles. 

Let θ be the angle of a regular decagon. Then, 

\(θ=(\frac{2\times10-4}{10})=\frac{8}{5}\)  right angles 

 \(\therefore θ=(\frac{8}{5}\times90)^°=144^°\)         [∴ 1 right angle = 90°] 

Also, \(\thereforeθ=(\frac{8}{5}\times100)=160^g\)      [∴ 1 right angle = 100^g ]

And \(θ=(\frac{8}{5}\times\frac{\pi}{2})^c=(\frac{4\pi}{2})^c\) 

[∴ 1 right angle = \((\frac{\pi}{2})^c\) ]