We are given the perimeter of a square. Since a square has all of the sides of the same length the perimeter is, therefore:
Where "l" is the length of the side. Solving for "l" by dividing both sides by 4:
Replacing the value of "P":
Solving the operations:
The length of the diagonal of a square is given by:
Replacing the value of the length we get:
Solving the operation:
Therefore, the length of the diagonal is 19.8 cm.