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单词 InterceptTheorem
释义

intercept theorem


Theorem.  If two intersecting lines are cut by parallel linesMathworldPlanetmath, the line segmentsMathworldPlanetmath cut by the parallel lines from one of the lines are proportional to the corresponding line segments cut by them from the other line.

The theorem may be condensed to the following form:

  • If a line parallelMathworldPlanetmath to a side (http://planetmath.org/TriangleMathworldPlanetmath) BC of a triangle ABC intersects the other sides in the points D and E, then the proportion equation

    BD:DA=CE:EA(1)

    is true.

The intercept theorem has been known by the ancient Babylonians and Egyptians, but the first known proof is found in Euclid’s Elements.

Proof.  The areas of triangles, which have equal heights, are proportional to the bases of the triangles; if the bases are equal, then also the areas are equal.  These facts are used in the

BD:DA=ΔBDE:ΔDAE=ΔCED:ΔEAD=CE:EA

of equalities. Q.E.D.

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更新时间:2025/5/4 6:23:27