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切线长定理:从圆外一点引圆的两条切线,它们的切线长相等,圆心和圆外一点的连线平分两条切线的夹角.如图1,
Tangent length theorem: A circle from the outside of the circle two tangent, tangent their length equal to the center of the circle and a little line bisecting the angle between two tangent as shown in Figure 1,