Answer:
See below. 
Explanation:
To find the distance between any two points, we can use the distance formula: 

We want to prove that the distance between a point P(x,y) and the origin is: 

Remember that the origin is at (0,0). 
So, let's prove it using the distance formula. Let's let the origin point (0,0) be (x₁, y₁) and let's let P(x, y) be (x₂, y₂). Substitute these values into the distance formula. This yields: 

Subtract: 

Simplify: 

This is the same equation as given. 
Q.E.D.