math

Not only that, but a triangle is 180°

therefore a triangle is half a circle.

Yes, it seems all polygons are circles in disguise :)

What a great discovery you have made   

You are saying that when you draw arbitrary radii of a circle, the angle sum at the center is 360 degrees. And when you draw all diagonals of a square, the angle sum at the center is also 360 degrees.

This is always true, because at any point, the angle sum at the same point is always 360 degrees. But as you say a triangle "is 180 degrees", I think you meant the interior angle sum of triangle is 180 degrees, which is half of 360 degrees. But this does not mean a circle is a square and a triangle is half a circle.