That depends how exactly the measurements of the sides are - and how exactly the "right angle" is really a right angle. If the measurements are exact, then, yes, you can also calculate the diagonal exactly - using Pythagoras' Theorem.
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Improved Answer:
No it's not possible to find the accurate length of the diagonal of any square using Pythagoras' theorem because the answer will always be an irrational number which can never be determined just like the value of pi in a circle.
It is the longest distance between two opposite corners of the square and it is possible to use Pythagoras' theorem to find the length of the diagonal
Use Pythagoras' theorem to find the length of the diagonal in the square
Using Pythagoras' theorem: 425
Using Pythagoras' theorem the diagonal is 16 times the square root of 2
If they are the dimensions of a rectangle then use Pythagoras' theorem to find the length of the diagonal.
60.28 is the length of the diagonal, got through Pythagoras' Theorem.
Using Pythagoras' theorem the length of the diagonal is 15 units
Yes Pythagoras' theorem can be used to find the interior diagonal of a cube for example.
It is the distance between opposite corners and it can be worked by using Pythagoras' theorem.
If it's the dimensions of a rectangle then by using Pythagoras' theorem its diagonal is 40
Using Pythagoras' theorem: 21.21320344 cm
By Pythagoras's theorem it is 15*sqrt(2) = 21.213 approx.