The square of a natural number is a four digit number, the thousand digit number is 4, and the five digit number is 5______ .

The square of a natural number is a four digit number, the thousand digit number is 4, and the five digit number is 5______ .

∵ the square of natural number is a four digit number, thousand digit number is 4, and ∵ 1002 = 10000, 92 = 81, ∵ this natural number can only be two digits, ∵ the number of digits is 5, ∵ 602 = 3600802 = 6400, ∵ its ten digit number may be 6 or 7, = 5625652 = 4225, ∵ its ten digit number is 6, ∵ this number is 65