Triangle BEF is similar to triangle DAF, [AAA]
Area of triangle BEF: area of triangle DAF =(5: 10)? = 1/4
The area of triangular DAF =4* the area of triangular BEF,
The area of the triangle is BAD = 10*9/2=90/2,
That is, the area of triangle ABF+the area of triangle DAF =90/2,
Area of triangle ABF +4* area of triangle BEF =90/2, ... 1)
The area of the triangle ABE =5*9/2=45/2,
The area of triangle ABF+ the area of triangle BEF =45/2,
Area of triangle ABF =45/2- area of triangle BEF, ... 2)
2) Substitute 1):
The area of the triangle BEF =(90/2-45/2)/3= 15/2.
Triangular area DEF = triangular bed area-triangular bed area BEF
=5*9/2- 15/2= 15.