=(n+7+n-5)(n+7-n+5)

= 12(2n+2)

=24(n+ 1)

Because n is a natural number.

So (n+7) 2-(n-5) 2 can be divisible by 24.

20.* a/b = c/d = k

∴a=bk c = Danish krone

∴a-c/b-d=bk-dk/b-d=k

a+c/b+d=bk+dk/b+d=k

∴a-c/b-d=a+c/b+d

The conclusion proves that

24. plant it on the hillside with a height of h meters ~

The hillside temperature is (22 hours ×0.55/ 100)℃.

① 16≤(22 hours× 0.55/100)

②(22 hours ×0.55/ 100)≤20

2≤(H×0.55/ 100)≤6

363.64≤H≤ 1090.9 1

25.a，a+ 1，a+2；

Satisfy a+(a+ 1)+(a+2)

So a

So A can take four groups: 0, 1, 2, 3,1* *.

So the answer is

0 12

123

234

345

26. Solution: If the number of people in the group is X, and the fees charged by the two travel agencies A and B are the same, the equation is obtained.

400+50(x-4)= 100×3 ^ 4x，

The solution is x = 8.

When the group number is 10, travel agency A charges 700 yuan < travel agency B charges 750 yuan, indicating that A is more favorable;

When the number of tours is 4, the charge of travel agency A is 400 yuan > the charge of travel agency B is 300 yuan, which means that travel agency B is more favorable.

To sum up, when the number of tours is greater than 8, the cost of travel agency A is more favorable;

When the number of tours is 8, the fees charged by the two travel agencies are the same.

When the number of people in the group is less than 8, the charge of B Travel Agency is more favorable.

(all right! ! ! ! ! I'm exhausted! ! ! )

(Please be sure to adopt ~ ~ ~! ! ! )