5. We know: $9=3 \times 3, 16=4 \times 4$, here, $9, 16$ are called "perfect squares". Among the first 300 natural numbers, if we remove all the "perfect squares", what is the sum of the remaining natural numbers?
5.【Solution】The squares not exceeding 300 are:
$1,4,9,16,25,36,49,64,81,100,121,144,169,196,225$,
256, 289, and their sum is 1785
The sum of the first 300 natural numbers is: $1+2+3+\cdots+300=\frac{1+300}{2} \times 300=45150$,
Thus, the sum of the remaining natural numbers is $45150-1785=43365$