Double-click on the movie to PLAY.

Click once to PAUSE.

Double-click again to return to the START of the movie.

NEXT PAGE

Here we raise BC by moving points B and C separately (left).

As B moves, the 'base' AC stays the same but the height decreases.

Then, as C moves, the 'base' AB' stays the same but the height decreases.

So the area decreases at each step.