What shape must the DECF of a quadrilateral be? 2 cutting?
When AC=24cm, BC=20cm, ∠ ACB = 45, please explore: how to cut the quadrangle DECF to maximize its area, and prove your conclusion; (2) folding?
Please use only two folds to determine the vertices d, e, c and f of the quadrilateral, so that it happens to be a diamond, and explain your folding method and reasons.