Length of the sides of a square

Pythagoras Theorem. H^2 = A^2 + B^2.

A = B, hence H^2 = 2xA^2.

ie A= The Square root of half H squared.

Thanks. It's a 19 inch computer monitor but it's square and not around 10 x 14 like most monitors.

Interesting... I don't think I've ever seen a square computer monitor.  It was typical to have a 4:3 ration between width and height, but not square.  Where did this come from?

https://charlotte.craigslist.org/ele/d/charlotte-computer-monitor-19-inches/6821518754.html

So mathmatically all monitors could be made square? The diagonal length could be 19 as is the case for the one from that link. Or the diagonal length could be 25, 40, etc. and still you could have a square.

Certainly.  You can pick any length and construct a square with that length as either a side or a diagonal.

If you want to use that shape as a computer monitor, you will have to think about the interface.  For example, to connect a square monitor to a standard 640 x 480 VGA card, you would have to discard some pixels or stretch all of them.  

Getting a circle to look round on a CRT usually required some tweaking of the aspect ratio.

I heard from that seller who has the monitor listed in that link above. They say the screen is 14.75 wide × 12 inches high and diagonal 19.