Prove the construction method used for constructing similar triangles of the given ratio of corresponding sides.
BC′C′C=31
Therefore, BCBC′=BC′+C′CBC′=1+C′CBC′=1+13=43, i.e., BC′BC=34
Also C'A' is parallel to CA. Therefore, △A'BC' ~ △ ABC.
So, A′BAB=A′C′AC=BC′BC=34