GMAT Club Tests | Re: M21-20

We can use an alternate method that will be less time consuming.
First we need to find the slopes of the 3 lines that form a triangle inside the circle.
Let the points be A(1,2), B(2,5), C(5,4). And the triangle will be ABC.
Hence the slopes will be as follows:
AB=3, BC=-1/3
Slope of AB*Slope of BC=-1
This implies that AB and BC are perpendicular to each other and AC is the hypotenuse of the triangle ABC.
We know that the diameter of a circle makes a right angle triangle inside the same circle.
Hence AC is nothing but the diameter of the circle.
Lenght of AC=√ (4-2)^2+(5-1)^2
=√ 20
=2√ 5
Option B is the correct answer.