A Triangle's Angles

The sum of the measures of the interior angles of any triangle is 180°.

To mirror an object, make sure the "Mirror object at point" tool is selected.  Then click the object to mirror, followed by the point.

How does this show that the angles add up to 180°?
Can you use this to find a formula for the area of the triangle?

Reflect each small triangle about its white point.

M = (-3.08, 0.3)
N = (6.8, 1.68)
a = Line(M, N)
D = Point(a)
b_2 = OrthogonalLine(D, a)
A = Point(b_2)
B = Point(a)
poly1, d, a_1, b_1 = Polygon(A, B, D)
C = Point(a)
poly2, d_1, b, c = Polygon(A, C, D)
E = Midpoint(A, D)
F = Midpoint(D, B)
G = Midpoint(C, D)
e = Line(E, b)
f = OrthogonalLine(G, b)
g = OrthogonalLine(F, a_1)
H = Intersect(e, f)
I = Intersect(e, g)
h = Segment(A, D)