PRINCIPIA · THEOREM

Similar triangles — corresponding altitude / median / angle-bisector ratios = similarity ratio

Dependencies: AA similarity, SAS / SSS similarity (SSS and SAS similarity tests).

Statement

Let ABCABC\triangle ABC \sim \triangle A'B'C' with similarity ratio kk, i.e.

ABAB=BCBC=CACA=k,A=A,  B=B,  C=C.\frac{AB}{A'B'} = \frac{BC}{B'C'} = \frac{CA}{C'A'} = k, \qquad \angle A = \angle A',\;\angle B = \angle B',\;\angle C = \angle C'.

Let hh, mm, tt denote, respectively, the altitude, median, and Angle bisector ⇔ equidistant from sides length from vertex AA in ABC\triangle ABC (each with foot on side BCBC); and hh', mm', tt' the corresponding cevians of the same kind in ABC\triangle A'B'C'. Then

hh  =  mm  =  tt  =  k.\frac{h}{h'} \;=\; \frac{m}{m'} \;=\; \frac{t}{t'} \;=\; k.

That is, every interior segment defined by similar triangles in a "corresponding" manner scales by the similarity ratio kk.

The corresponding altitude / median / angle-bisector ratios in similar triangles all equal the similarity ratio k

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