Cho tam giác ABC vuông cân tại A, đường trung tuyến AM(M thuộc BC). Lấy điểm P trên cạnh AB sao cho P khác A và B, vẽ đường thẳng song song với BC và AM, hai đường thẳng này cắt AM và BC lần lượt ở N và K. a) Chứng minh PNMK là hình bình hành b) Trên tia đối của tia MA lấy điểm D sao cho MD=MA. Chứng minh ABDC là hình chữ nhật c) Chứng minh PK+PN=AD/2

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