Question 18, if we add a 0 to the first two 5s of the first equation. We can find that the law on the left of the equal sign of the above four equations is 1: 05 * 05, 2: 15 * 15, 3: 25 * 25, 4: 35 * 35, while the law on the right of the equal sign is1:0 *.

So we can sum up as follows. .

(n-1) *10+5 * (n-1) *10+5 = (n-1) * n *100+25n is a positive integer.

I hope it helps you.