The length t is the length of the red arc.
Draw a perpendicular from the point to the
x-axis. Label it Q, and label the origin O. Then draw the radius OP.
The radius of the unit circle is 1. {In green below].
In geometry, you learned that a central angle is measured by the
arc it subtends. So the central angle is also measured t.
Triangle OPQ is a right triangle.
The side opposite angle t is PQ.
Also PQ is the y-coordinate of P, so PQ = 8/17.
The side adjacent to angle t is OQ.
Also OQ is the x-coordinate of P, so OQ = 15/17.
The radius OP is the hypotenuse, so OP = 1.
<-- the y-coordinate of P
<-- the x-coordinate of P
Edwin