ABCD is a square,
∴AD=AB,∠D=∠ABC=∠ABE=90
AD = AB，BE=DN，∠D=∠ABE=90
∴△ABE≌△ADN(SAS)
∴S△ABE=S△ADN
AN=AE，∠DAN=∠BAE
∠∠ Dan+∠ Bam = ∠ Bud-∠ Mann =90 -45 =45
∴∠ BAE+∠ BAM = 45, that is ∠ EAM = ∠ Man = 45.
AE = AN，AM=AM，∠EAM=∠MAN
∴△AMN≌△AME
Then s △ AMN = s △ AME = s △ ABM+s △ Abe = s △ ABM+s △ ADN.