Outer corner property. Formulate and prove.

The outer angle of a triangle is equal to the sum of the two remaining angles of the triangle that are not adjacent to this outer angle, that is:
angle d = angle a + angle c.
Let ABC be an arbitrary triangle. a, b and c – angles of triangle ABC, d – external angle at angle b.
Angles b and d are adjacent, since they have a common vertex, side and together form a straight line, then:
angle b + angle d = 180 degrees;
d = 180 – b.
By the theorem on the sum of the angles of a triangle (the sum of all interior angles of any triangle is 180 degrees):
angle a + angle b + angle c = 180 degrees;
a + c = 180 – b.
Since d = 180 – b and a + c = 180 – b, then:
d = a + c.
Q.E.D.