Find the greatest four-digit number that is a perfect square.

9801

We all know that the square of 90 is 8100 and square of 100 will be 10000. We need to find the greatest four-digit number that is a perfect square. (100)² gives a 5-digit number. Thus, the number which gives highest  4 digit number that is a perfect square will be 100 minus 1 = 99. Thus, square of 99 will be the answer.

9801 = (99)²