If the dog were on a 15-ft leash tied to a stake in the ground out in the middle of a field, then he could cover every blade of grass in a 15-ft circle. The area of a circle is (pi R²), so he would have free range over (pi · 15²) square feet of area.
Sadly, he's not out in the middle of a field. He's tied to the corner of the house. So he can't cover the whole circle, because the house blocks a quarter of the circle. The dog only has access to
so angles 231, -129 and 591 corresponds to the same terminal while 51 is not related to the particular quadrant . so 51 is not co-terminal with the other three angles .