Which of the points (x, y) does NOT lie on the unit circle?

  a)  P(1/2, 1/2)

Step-by-step explanation:

If the point does not lie on a coordinate axis, then the sum of the coordinates must be greater than 1. (This is a consequence of the triangle inequality and the fact that the hypotenuse of the right triangle is 1.)

This fact lets you readily identify (1/2, 1/2) as not being on the unit circle.