Knowledge point: The sum of the other two angles of the figure-eight triangle is equal except the diagonal.

Prove: ∫≈ 1+∠B =∠3+∠M, ∠4+∠D=∠2+∠M,

∴∠ 1+∠4+∠B+∠D=∠3+∠2+2∠M，

∵∠ 1=∠2,∠3=∠4,

∴2∠M=∠B+∠D，

∴∠M= 1/2(∠B+∠D)。