Given:
The perimeter of a square is 54cm.
To find:
The length of a diagonal, rounding to the nearest tenth.
Solution:
Let a be the length of each side of square.
We know that, perimeter of a square is


Divide both sides by 4.


We know that, diagonal of a square is



Rounding to the nearest tenth, we get

Therefore, the length of diagonal is about 19.1 cm.