Question 814819
Let y = size of unknown side.
{{{40^2+y^2=41^2}}}, because the diagonal is the hypotenuse of the triangle with the legs, 40 and y.


{{{y^2=41^2-40^2}}}
{{{y=sqrt(41^2-40^2)}}} which is simplifiable as
{{{y=sqrt((41-40)(41+40))}}} --------- Do you recognize why?
{{{y=sqrt(1*81)}}}
{{{y=9}}}


Solved for y is the main missing piece to finding the answer to the actual question, which I now leave for you to finish.