High school math. How to find the area of triangle DEF?

∵ef∨ab, and∵ square ABCD, ∴EF∥DC, ∴C, D, E and F are on the same plane.

AB⊥BC of ∴EF⊥BC, BFC of ∴EF⊥ FB, ∴EF⊥FC,

The distance from point ∴D to EF is equal to two parallel lines EF and CD =FC= radical number 2, and the hypotenuse of isosceles right angle △BFC =2.

∴△def∴△ Distance from the point of area ef * d to ef/2 =( 1* root number 2)/2 = 1/2* root number 2,

∵BF⊥ plane CDEF, ∴ height of triangular pyramid B-DEF =BF= radical number 2,

∴ volume of tetrahedron B-DEF = 1/3* 1/2* radical number 2* radical number 2= 1/3.