Traditional Culture Encyclopedia - Photography and portraiture - Two squares with side length a are joined to form a right dihedral angle AB-CD-EF. Find the cosine of the angle between AC and DF.

Two squares with side length a are joined to form a right dihedral angle AB-CD-EF. Find the cosine of the angle between AC and DF.

As shown in the figure, make a cube and find the degree of ∠CAG, which is the angle formed by AC and DF. This will solve the key problem.

In the figure, the three diagonals are equal, then △ACG is an equilateral triangle. For reference only, it is equivalent to translating DF to AG and finding the degree of ∠CAG, which is the degree of the angle formed by the out-of-plane straight line AC and DF.

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The final result is 1/2