∴∠A+∠B+∠C+∠D+∠E
=∠A+∠ 1+∠C+∠D+∠2
= 180 (the sum of the interior angles of the triangle is 180)
Connect the CD, then ∠B+∠E=∠ 1+∠2 (equal to 180- a pair of diagonal).
∴∠A+∠B+∠C+∠D+∠E
=∠A+∠ 1+∠C+∠D+∠2
= 180 (the sum of the interior angles of the triangle is 180)