Consider the figure below:
If AB = 5 cm; QR = 7cm; then find the value of AC (in cm.), if the perimeter of △ABC is 18cm.
In the given figure:
In ΔABC and ΔPQR
AB = PR = 5 cm
BC = PQ
AC = QR = 7 cm
Therefore, ΔABC≅ΔPQR by SSS criterion.
Since perimeter = 18cm
⇒ AB + AC + BC = 18 cm,
⇒ BC = 18 - 5 - 7 = 6 cm.
Therefore,
AB = PR = 5 cm
BC = PQ = 6 cm
AC = QR = 7 cm
So, the answer is 7 cm.