Hey there! :D
So, basically, you could say that "x" is equal to both of the legs.
We also know that in 45-45-90 triangles, x*√2= the hypotenuse
Given this information, if we were to divide 10 by the √2, we would get:
10/√2= 7.071.....
5√2= 7.071....
5√2 is the answer.
I hope this helps!
~kaikers
