Find the side AC of triangle ABC if its P = 21 cm, AB = 7 cm, BC = 9 cm. What is AC?

To find one of the sides of a triangle, you need to subtract the lengths of the other two sides from the perimeter.

Let’s write down the formula for finding the perimeter of a triangle.

P = AB + BC + AC.

Let us express the desired side of the AU and rewrite the expression.

AC = P – AB – BC.

Let us substitute the values P = 21 cm, AB = 7 cm and BC = 9 cm, given according to the condition, and then we will make the calculation.

AC = 21 cm – 7 cm – 9 cm.

AC = 5 cm.

Thus, the unknown side of triangle ABC is 5 cm.

Answer: AC = 5 cm.