I use an easy(for my learners) method that makes sense to understand the negative and positive angles in CAST diagram. The method works like this, if you are given a trig ratio sin (-225). Take 360-225 = 135. That means sin (-225) = sin 135. It sounds logical as the revolution (by convention) is 360 degrees, hence an anticlockwise + clockwise measure = 360 degrees, as two such angles share a co-terminal, which is actually one and the same position in terms of CAST diagram.
I was very pleased to let my learners get an easy approach to this abstract concepts especially with limited technology.
I wish I could comment on this.
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