Find the greatest number of 4 digits which is a perfect square.

The greatest number with four digits is 9999. To find the greatest perfect square with four digits, we must find the smallest number that must be subtracted from 9999 in order to make a perfect square. For that, we have to find the square root of 9999 by the long division method as shown below:

We must subtract 198 from 9999 to make a perfect square:

9999 − 198 = 9801

Hence, the greatest perfect square with four digits is 9801.