Analysis: let NF⊥AD, vertical foot f, connect DD ′, nd ′, and get DD ′⊥ Mn according to the properties of graph folding. Prove △ dad ′∞△ DEM first, then △ nfm △ ade, and then use the knowledge of Pythagorean theorem to find the length of Mn.

Answer:

Solution: NF⊥AD, vertical foot f, connection DD ′, nd ′,

∵ Fold the square paper ABCD in half, so that the D point falls on the D' point on the AB side, and the crease is MN.

∴dd′⊥mn，

∠∠A =∠DEM = 90，∠ADD′=∠EDM，

∴△dad′∽△dem，

∴∠dd′a=∠dme，