A square with a 30-inch diagonal measurement has sides of 21.21 inches in length.
Use Pythagoras. The diagonal, and two sides of the square form a right angled triangle. So if each side of the square is x cm long, the diagonal is x*sqrt(2) cm long.
Each of them is 14.14 inches long. (rounded)
14.142 cm (rounded)
If the area of the square is 200, then the side is sqrt(200).The length of the diagonal is sqrt(200 + 200) = sqrt(400) = 20 meters.
162+162=diagonal2 256+256= 512 sqrt512 i.e. 16rt2
Using Pythagoras' theorem each side will be 6.363961031 units in length.
There is no standard measure. The monitor that I am using now is 71 cm long. Also, like TV screens, the size of a monitor is the length of its diagonal.
The diagonal is 14.142 feet.
The diagonal is 33.941 units.
The diagonal is about 16.97 inches long.
If the only known fact is the length of the diagonal then the width and length of the rectangle CANNOT be determined. The diagonal could be that of a square, or of a rectangle that is very long but quite narrow. Consequently at least one more fact is required such as; the dimension of either the length or the width, or the angle that the diagonal makes to the base of the rectangle or even the area of the rectangle.