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题目如图1,在正方形ABCD中,N点为BC边上的任一点,直线CM平分正方形的外角∠DCK．若∠ANM= 90°．求证:AN=NM．
The title is shown in Fig. 1. In the square ABCD, N is any point on the BC side, and the straight line CM divides the outer corner of the square ∠ DCK. If ∠ANM = 90°. Proof: AN=NM.