The left hand side adds angles 1,2, 5, and 6 giving the sum of the summit angles (the angles that are acute, the angles where the defect occurs.)
The left hand side adds angles 1,2, 5, and 6 giving the sum of the summit angles (the angles that are acute, the angles where the defect occurs.)
The right hand side adds angles 2,3, 4, and 5 giving the sum of the interior angles of the triangle.