Question

find the greatest number of 6 digit which must be a perfect square

Answer 1

we know that,
10002 = 1,000,000 which is only 1 more than 999,999. so the greatest six digit number which is perfect square = (1000 -1)2.

Answer 2

Let x² be the largest six-digit perfect square. The largest six-digit number is 999,999, so.
x² ≤ 999,999.
You know that 1,000² = 1,000,000, which is only 1 more than 999,999, so x² must be the next smaller square, which is (1,000 - 1)².

hope it's help you