n是正整數且4≤根號n≤9,則如何證明n有66個？

4 ≤ 根號n ≤ 9
平方後 16 ≤ n ≤ 81
n為正整數，所以n有 81-16+1=66個。

兩邊平方得16<=n<=81,所以n有81-16+1=66個解

divide into two parts: square root n>4 and square root n<9
(square root n)^2>4^2
n>16
(square root n)^2<9^2
n<81
so, 16<n<81
81-16=65 plus the highest number(81) =66