中二數學！！！！！

Let the size of each exterior angle be x, the size of each interior angle be 9x

x + 9x = 180° ( adj. ∠s on st.line )

10x = 180°

x = 18°

The size of each exterior angle is 18°

Let the no of sides of this polygon be y

y( 18° ) = 360° ( sum of ext. ∠ of polygon )

y = 20

[(a-2)*180]/a=(360/a)*9


唔知correct or wrong!!!