Trên đường thẳng AA’ lấy điểm O. Trên 1 nữa mặt phẳng bờ AA’ vẽ tia OB sao cho Góc AOB = 45 độ . Trên nữa mặt phẳng còn lại vẽ tia OC sao cho góc AOC = 90. Độ a) gọi OB’ là tia phân giác của góc A’OC . Chứng tỏ rằng 2 góc AOB và A’OB’ là 2 góc đối đỉnh b) trên nửa mặt phẳng bờ AA’ có chứa tia OB vẽ tia OD sao cho góc DOB = 90 độ. Tính góc A’OD.

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