Question

If the point (1, 5) lies on a circle whose center is at the origin, what is the equation of the circle?

Answer 1

In this case just change the variables by the numbers. (1,5)
x=1 y=5
1^2 + 5^2 = 26
1+25=26
26=26 *this one is the answer...
TRUE

x2 + y2 = 16
1^2 + 5^2=16
1+25=16
26=16
False

x2 + y2 = 13
1^2 + 5^2=13
1+25=13
26=13
False

x2 + y2 = 11
1^2 + 5^2=11
1+25=11
26=11
False

So we can say that the first one is the answer.