Consider the triangles AA1B and CC1B.
∠ABA1 = ∠CBC1, since they are vertical.
∠AA1B = ∠CC1B, since they are direct by the condition of the problem.
Therefore, according to the first sign of the similarity of triangles, these triangles are similar.
Then, by the definition of such triangles:
AB / BC = A1B / C1B
We transform this equality:
AB / A1B = BC / C1B
Consider the triangles A1BC1 and ABC.
∠ABC = ∠A1BC1, since they are vertical.
Then, according to the second sign of similarity, these triangles are similar.
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