why sin90 is equal to 1

Here is a video.

I won't rave about it - it is not the way I would usually explain it - but you might find it good.

https://www.youtube.com/watch?v=3sWN5mdF_6U

I just found this one - I think it is better. START HERE.

http://www.mathsisfun.com/geometry/unit-circle.html

sin(90) is equal to 1 because you are taking the opposite of the angle over the hypotenuse, and in this situation it just happens to be the same number. Any number over itself equals 1.