Question 559960
The diagonal forms a triangle with two of the sides.

You can either use the rules for a 45-45-90 triangle (which this will create) or you can use the Pythagorean theorem (where the rules for the 45-45-90 triangle comes from).

So Pythagorean theorem is {{{a^2+b^2=c^2}}} but in this case a=b since all the sides of a square are the same. So we have {{{a^2+a^2=c^2}}} which is {{{2a^2=c^2}}}

{{{sqrt(2a^2)=c}}}

{{{sqrt(2)*a=c}}}

Which is the rule for a 45-45-90 triangle. If the two sides are x then the hypotenuse is {{{sqrt(2)*x}}}

So we have {{{sqrt(2)*13.5}}} So the answer will be around 19.09