The side length of square ABCD is a, PA ⊥ plane ABCD, PA = a, m, n are the midpoint of PD, Pb respectively, then the cosine value of the angle formed by the line am and cn is

The side length of square ABCD is a, PA ⊥ plane ABCD, PA = a, m, n are the midpoint of PD, Pb respectively, then the cosine value of the angle formed by the line am and cn is

Let me try... Two methods are given, I hope LZ can add points to connect dB, make AE / / DB through a, cross CB extension line to e, take the midpoint of AE F, connect NF, Mn ∵ m, n are the midpoint of PD, Pb ∵ Mn / / = 1 / 2bd ∵ AF = 1 / 2ae / / = 1 / 2bd ∵ Mn / / = AF ∵ mnfa is parallelogram ∵ AM / / = FN, the angle between line am and CN