4. The center of a circle has a center of (5, 1), and an endpoint of the diameter of the circle is (2,5). What are the coordinates of the other endpoint of the diameter?

Let the other end point be (x,y)
THe two endpoints are in a straight line where the Center would be the MIDPOINT of them.
Thus, we can say
(2, 5) ----------- (5,1) ----------- (x,y)
We can use midpoint formula here, which is:
Midpint Formula:
[tex](\frac{x_1+x_2}{2}),(\frac{y_1+y_2}{2})[/tex]So,
[tex]\begin{gathered} \frac{2+x}{2}=5 \\ 2+x=10 \\ x=10-2 \\ x=8 \end{gathered}[/tex]and
[tex]\begin{gathered} \frac{5+y}{2}=1 \\ 5+y=2 \\ y=2-5 \\ y=-3 \end{gathered}[/tex]Hence the midpoint is (8, -3)