how $x= \cos\theta$ and $y= \sin\theta$? I'm learning Trigonometry right now and at current about trigonometry functions. I'm little bit confused right now in a section of the chapter. Please have a look at the image. 
I didn't get the second paragraph .How $x= \cos \theta$ and $y = \sin \theta$? Can we name the sides of the triangle with same name as of coordinate sides like in the figure the triangle is having x and y value? 

EDIT- I also want to know why radius of the circle is taken unity and not more than that. For example - 2 or 3 or 4. There will be any difference or the conclusion will be same? 

$$x=r\cos\theta$$
the radius  here is $r=1$
$$x=\cos\theta$$
$$y=r\sin\theta$$
$$y=\sin\theta$$
