cant understand this solution in certain points..

on a circle with radius 1 there are two points one is static the other is

distributed evenly on the circle (i dont know the proper english term)

we sign X to be the hypotenuse on the circle

find the density function of the variable X

?

solution(i partialy understood it)the questions come after the photo

http://i46.tinypic.com/n3t35v.jpg

they say that we take x to be an angle from o til pi/2

we scan only the northern hemisphere because its symmetric.

what is the curv graph represents

what is t?

what is the meaning of y<=t?

why if they say that x should go from o til pi/2?

in the probability function they put x fro 0 till arcsin t/2 where t goes from

0<t<=2

?

after this step they did the derivative to find the density function

i understand that step