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圆既是轴对称图形,又是中心对称图形,直径所在直线是它的对称轴,直径的中点是对称中心.与弦垂直的直径,平分弦及弦所对的弧;直径对的圆周角是直角.可见直径是圆中最活跃的因素,它是沟通弦、角、弧关系的桥梁.解题中,若能从直径入手,可突破难点,化难为易.(1)在题设或结论中涉及弧的中点、弦的中点,解题时往往考虑构造“垂径”.(2)在题设、结论中涉及直径或直角,解题时要关注直径对的圆周角.
The circle is both an axisymmetric figure and a centrosymmetric figure, the line where the diameter lies is its axis of symmetry, and the midpoint of the diameter is the center of symmetry. The diameter perpendicular to the string bisects the arc to which the string and the chord correspond; Right angle. Visible diameter is the most active factor in the circle, it is to communicate the bridge of chord, angle, arc. Problem-solving, if you can start from the diameter, you can break through difficulties, difficult for the easy. (1) In the middle of the arc, the mid-point of the string, often consider the construction of the “vertical” (2) in the title, the conclusion relates to the diameter or right angle, the problem-solving time to pay attention to the diameter of the circle angle.