Hey,here is the answer to ur question......!!!! Since it is a square and all angles =90 Therefore to Find length of the diagonal we have to use Pythagoras theorem....! So, (AC)^2=(AB)^2+(BC)^2 =>(AC)^2=(11)^2+(11)^2 =>(AC)^2=121+121 =>(AC)^2=242 =>AC=root242 =>AC=11root2 =>length of the diagonal =11root2 Hope this helps u....!!!!