What is this formula in python?

Question

Python

there is a code:

import math
 
print("Введите координаты точки: ")
x = float(input("x = "))
y = float(input("y = "))
print("Введите центр окружности и его радиус: ")
r = float(input("R = "))
xk = float(input("x(k) = "))
yk = float(input("y(k) = "))
 
 
if (x - xk)**2 / r**2 + (y - yk)**2 / r**2 < 1:
    print("Точка принадлежит кругу")
else:
    print("Точка не принадлежит кругу")
a=input("")

It is necessary to explain:
1) what it means 2) explain why less than 1 (<1)
if (x - xk)**2 / r**2 + (y - yk)**2 / r**2 < 1:

Answer 1

The circle equation ω (A; R) has the form (x – a)^2 + (y – b)^2 = R^2, where a and b are the coordinates of the center A of the circle ω of radius R.

Answer 2

Less than 1 because the formula considers relationships. The ratio of the square of the difference in coordinates to the square of the radius. And their sum, as a result, is compared with 1. Roughly speaking, if the coordinates of the point coincide with the center, the sum will be zero, anything less than 1. The farther from the center and closer to the circle, the closer to 1 the result of the formula. If the point is on the circle, the formula will give 1. The point outside the circle - the formula will give more than 1.
So they compare with 1 through if.

Answer 3

The usual formula for a point belonging to a circle