Question 262368
Remember that the diagonal of a square splits the square into 2 right triangles.  So we can use Pythagorean's theorem.  Remember that 

{{{a^2 + b^2 = c^2}}}

where c is the hypotenuse.  So the diagonal is the hypotenuse here, and thus c = 10.  Now, since we are dealing with a square, all the sides are the same length, so a = b.  So we have:

{{{a^2 + a^2 = 10^2}}}
{{{2a^2 = 100}}}
{{{a^2 = 50}}}
{{{a = sqrt(50)}}}
{{{a = 5sqrt(2)}}}