The perimeters of two similar triangles ABC and PQR are respectively 18 cm and 12 cm. If PQ =5 cm, find AB.

The ratio of the corresponding sides of similar triangles is same as the ratio of their perimeters.

Since ∆ ABC ~ ∆PQR

$$\begin{gathered} \frac{AB}{PQ}=\frac{18}{12} \\ \frac{AB}{5}=\frac{18}{12} \\ AB=7.5cm \end{gathered}$$