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平面几何中的圆幂定理是相交弦定理、切割线定理及割线定理的统称.相交弦定理:圆的两条相交弦,每条弦被交点分成的两条线段长的积相等.如图1所示,AP·PB=DP·PC.切割线定理:从圆外一点引圆的一条切线与一条割线,切线长是这一点到割线与圆的两个交点的线段长的等比中项.如图2所示,PA2=PC·PD.割线定理:从圆外一点引圆的两条割线,这一点到每条割线与圆的交点的两条线段长的积相等.如图3所示,PA·PB=PC·P
The theorem of circular power in plane geometry is the general term of the intersectional string theorem, the theorem of the cutting line and the secant theorem. Intersection Theorem: Two intersecting chords of the circle, each of the two products is equal in length. 1, AP · PB = DP · PC. Cutting Line Theorem: A tangent and a secant drawn from a point outside the circle, the tangent length is the ratio of this length to the length of the line segment where the two intersections of the secant and the circle are long The middle term, PA2 = PC · PD, is shown in Figure 2. Secant Theorem: The product of two secant lines that circle a point outside the circle to the length of the two line segments where each secant intersects the circle is equal As shown in Figure 3, PA · PB = PC · P