How Do You Know The Angle Of An Arc?

           To know the size of an arc is quite simple. Firstly draw an arc with your geometric compass. See that the instrument that you are using is correct and proper. Then you draw perpendicular lines at both the end points of the arc. Mark them as line AB and line PQ. Extend those lines so that they join at one given point.

           Name that intersecting point O. Then take a protractor and keep it in line with any one side of the lines drawn that make an angle. Keep the centre of the angle that is point O at the centre of the protractor and keep the line AB at zero degrees of the protractor. The line which correlates to any of the lines on the protractor marks the angle which is numbered on the protractor.

           You can always us the inscribed angle theorem and all the other arc angel theorems to measure your angel mathematically.