Find the diagonal of a rectangle with sides 12 cm and 9 cm.

From the condition, we know that the rectangle is given by two sides 12 cm and 9 cm.

In order to find the length of the diagonal, consider a right-angled triangle formed by the adjacent sides of the rectangle (they are legs) and the diagonal (it is the hypotenuse).

We need to find the length of the hypotenuse, for this we will use the Pythagorean theorem.

The square of the hypotenuse is equal to the sum of the squares of the legs.

a ^ 2 + b ^ 2 = c ^ 2;

c ^ 2 = 12 ^ 2 + 9 ^ 2;

c ^ 2 = 144 + 81;

c ^ 2 = 225;

c = 15 cm length of the hypotenuse.

Answer: 15 cm hypotenuse.