Could someone tell me the proof on why sin90= 1?

Answer 1

The key to understanding the first answer is that if you draw a circle and draw it's radius, you can create a triangle from it back to the center. I provided a link below that shows a graphical explanation of how you define sine and cosine using a circle at the origin with a radius of 1.

Answer 2

You've probably learned that sin is y, cos is x, and tan is y/x. so, if you look on the unit circle at 90 degrees, you will see that the corresponding point is (0,1). remembering that sin is y, look at the y spot of that coordinate. it's 1!