Double-click on the movie to PLAY.

Click once to PAUSE.

RELOAD the page or double-click to return to the START of the movie.

Method B1

Here, as well as rotating DC, we rotate a line parallel to DC that goes through the centre of rotation.

What angle does this new line turn through? So what angle does chord DC turn through?

On the next page, we label the angle that the line parallel to DC turns through and we show its size.