Hi Bunuel - My confusion is on the wording of the question- " If p is a point on the circle..." this is in fact saying that p has to be on the "outlined" circle and NOT for example inside the circle? If we choose p to be in the origin, 0,0 we would have another answer. But from what I am reading no one is considering P to be "inside" the circle. Are there any key clue to rule out points inside the circle?
Sallyzodiac
A certain circle in the xy-plane has its center at the origin. If P is a point on the circle, what is the sum of the squares of the coordinates of P?
1) The radius of the circle is 4.
2) The sum of the coordinates of P is 0.
Say the coordinates of P are (x,y), then the question asks about the value of x^2 + y^2.
In an x-y Cartesian coordinate system, the circle with center (a, b) and radius r is the set of all points (x, y) such that:
\((x-a)^2+(y-b)^2=r^2\)

This equation of the circle follows from the Pythagorean theorem applied to any point on the circle: as shown in the diagram above, the radius is the hypotenuse of a right-angled triangle whose other sides are of length x-a and y-b.
If the circle is centered at the origin (0, 0), then the equation simplifies to:
\(x^2+y^2=r^2\).
According to the above, the first statement of the question gives the direct answer: x^2 + y^2 = r^2 = 4^2. The second statement is not sufficient.
Check more here:
math-coordinate-geometry-87652.htmlHope it helps.