设D在AC上,CD=X,则AD=4-X,
∵CDEF是矩形,
∴DE∥BC,
∴ΔADE∽ΔACB,
∴AD/AC=DE/BC,
(4-X)/4=DE/2,
DE=1/2(4-X),
∴S矩形CDEF=CD*DE
=X*1/2(4-X)
=-1/2(X^2-4X+4-4)
=-1/2(X-2)^2+2,
∴当X=2时,S最大=2.
设D在AC上,CD=X,则AD=4-X,
∵CDEF是矩形,
∴DE∥BC,
∴ΔADE∽ΔACB,
∴AD/AC=DE/BC,
(4-X)/4=DE/2,
DE=1/2(4-X),
∴S矩形CDEF=CD*DE
=X*1/2(4-X)
=-1/2(X^2-4X+4-4)
=-1/2(X-2)^2+2,
∴当X=2时,S最大=2.