∵(BD+CD)/BD=(B′D′+C′D′)/B′D′{合比定理}→BC/BD=B′C′/B′D′→
BD/B′D′=BC/B′C′{移项}=3/2;
∴△ABD∽△A′B′D′ {已知∠B=∠B′;AB/A′B′=3/2}.