Then ∠C+∠D=∠CTE,

And ∠CTE+∠E=∠BMF.

That is ∠C+∠D+∠E=∠BMF.

rule

Angle A+ angle ABC+ angle C+ angle D+ angle DEF+ angle F.

= Angle A+ Angle ABC+ Angle BMF+ Angle F (these four angles are the quadrilateral of ABMF)

=(4-2)× 180

=360