1. Because the number under the radical sign must be greater than or equal to 0, 18-n≥0, then n≤ 18, and n is a natural number, that is, n≥0.

Since 0≤n≤ 18, only 0, 18-n ≤ 18 (see) has an integer square root, that is to say, 18-n = thus deduces that n= 18,/kloc-0.

2. Because n is a positive integer, n≥0 cuts n into integers, and when the testable value is n=0, it meets the meaning of the question. The answer is n=0.