The perimeter of a square is 84. Fine the length of a diagonal

Let's look at this again......if the perimeter is 84, each side is 21. But that's not the length of a diagonal. The length of a diagonal is √2 times as long as a side. So the diagonal length is 21√2.

What is a diagonal?

The diagonal divides the square into 2 isoseles right triangles.  It is like a line dividing the square (or any shape)

The length is 21.

Thank you!

You're welcome!

Yes thank you CPhill.  After I looked at it again I relised I needed to add that :)