Step 1: Investigate the given question and write the given values

            NOTE: The two triangles are similar

            EXAMPLE: ∆ABC ~ ∆PQR

                          Perimeter of △ABC = 20 cm.

                         Perimeter of △PQR = 30 cm.

                        One side of the first triangle QR = 12 cm, BC = ?

Step 2: Utilize the similarity principle to solve this  

            NOTE:  If two triangles are similar then ratio of the perimeters of the

              triangles is equal to the ratio of their corresponding sides.

                         \frac{perimeter of ABC}{perimeter of PQR} =   \frac{BC}{QR}

Step 3: Substitute the known values and simplify  

              EXAMPLE: \frac{20}{30} = \frac{BC}{12}

                                   \frac{20 * 12}{30} = BC

                                              BC =  8 cm            

            So required value of the corresponding side is 8 cm