Similar triangles has many judging theorems. Some appear in the textbooks of junior high school mathematics for nine years, and some do not appear in the textbooks, so we need to dig them ourselves. The following two triangles are basically used to discuss similar triangles's judgment theorem. 1, the original judgment theorem is to judge according to the definition, that is, triangles are equal and all three sides are proportional. That is, ∠A=∠A', ∠B=∠B', ∠C=∠C', AB: A 'B' = BC: B 'C' = CA: C 'A' = K. The ratio of this ratio is again If the similarity ratio of △ABC and △A'B'C is k, then the similarity ratio of △A'B'C and △ABC is1/k. Of course, we basically don't need to define judgment, but as knowledge, you must understand it. 2. Then there is the judgment theorem based on parallel lines, that is, if two triangles have two sides on the same straight line and the third side is parallel to each other, then the two triangles are similar. There are two situations, as follows: the original text of the textbook is to cut a straight line parallel to one side of the triangle on the other two sides (or extension lines), and the triangle formed is similar to the original triangle. So as long as there is DE//BC, we can determine △A[granite-brazil.cn].

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