what is square of 81?

Thanks for that answer, I made exactly the same mistake recently!

$${{\mathtt{81}}}^{{\mathtt{2}}} = {\mathtt{6\,561}}$$

because

$${\mathtt{81}}{\mathtt{\,\times\,}}{\mathtt{81}} = {\mathtt{6\,561}}$$

for this problem, the square root of 81 is 9 and -9 because: 

9 x 9 = 81 =) 

AND 

-9 x -9 = 81 =D (when you multiply two negative numbers you also get a positive) 

FINAL ANSWER: 

9 and -9