Hi, thanks for replying. You mentioned that the definition of cosine, adjacent divided by hypothenuse, can only be used on right triangles. When we are at pi/2 on the unit circle, the x and the y axis form a right triangle and we have an angle that is pi/2 or 90 degree between the x and the y axis. We should be able to divide the length of the side adjacent to 90 degree which is the x axis starting from the point (0,0) to the point (1,0), by the hypothenuse of that triangle . I would include an image to explain what I have just said but I don't see the upload option.
We all know that the cosine of pi/2 is 0 because it is the x coordinate of the point on the unit circle when we move counter clockwise from 0 degree to pi/2. How do you prove that cos of pi/2 equals zero using the law of cosine which is adjacent divided by hypothenuse. I figure since I was able to use the law of sine, law of cosine, and similar triangle to find coordinates of other points on the unit circle, I'd be able to prove that cosine of 90 degree or pi/2 equals to zero as well. However it doesn't look like it is possible. Please reply if you can show how this can be done, thanks.