A watermelon is cut 4 times and becomes 9 and a half type .

Wells, of course.

Isn't it?

I guess the tic-tac-toe mentioned by a few people upstairs.

To cut a large watermelon into eight pieces, three knives are used. Here's one possible cut:1

First, cut the knife vertically from the top of the watermelon, dividing it into two equal hemispherical sections. 2.Next, the second knife is cut horizontally into the watermelon from the cut, dividing it into two equal fan-shaped sections.

3.The third knife should be perpendicular to the incision, cut from the side, and also divide it into two equal fan-shaped parts. After these three knives, you will get eight pieces of watermelon, four of which are hemispherical and the other four are fan-shaped.

Note that when cutting the third knife, you need to make sure that Zhiju cuts the fan into two equal parts to ensure that you get eight pieces of watermelon of equal size.

How do you cut a watermelon into 9 pieces with 4 knives? Here's how to cut a watermelon into 9 pieces with 4 knives.

01 If the green circle is a watermelon, the red pen represents the knife.

02 Use a knife to cut the watermelon horizontally.

03 Use the knife again to cut the bottom of the first knife horizontally.

04 Then use the knife to cut vertically, through the line of the two horizontal knives.

05 Finally, from the left end of the second horizontal line to the right of the vertical line, make another diagonal cut, so that the watermelon is cut into nine pieces.

First, cut a whole watermelon horizontally and turn it into two pieces.

Then put the two watermelons together and cut them vertically into eight. Cut one of them open, and the watermelon becomes nine pieces. Three watermelons and nine watermelons.

Summary. How to cut a watermelon into 13 pieces with 5 knives.

Cut like this, then cut another knife in the cross-section 2 x 7 = 14

2+(n-1)(n^2+n+6)/6

This is just a conjecture and must be supported by proof by mathematical induction. After inspection, the 4 planes can only divide the space into 15 parts (3 planes can be divided into 8 spaces, but the 4th plane cannot divide all 8 planes into 2 parts), so the answer given by the second floor is wrong.

We know that n straight lines can be divided into 1+n(n+1) 2 parts at most, and I think the number of n planes should have something to do with it.

n Straight line is divided into planes and planes are divided into spaces.

We see that the number in the second column is equal to the sum of the number above it and the number to its left, and the number in the third column is equal to the sum of the number above it and the number to the left, and from this, I end up with 2+(n-1)(n 2+n+6) 6

There is also a speculation:

Cut a watermelon into 3 knives and cut it into up to 6 pieces.

Cutting 4 knives can cut up to 11 pieces, 5 knives can cut up to 17 pieces, and 6 knives can cut up to 24 pieces,..The formula is: a watermelon can be cut into up to 1 + (1 + 2 + 3 +...).n) = 1 + (n + 1) 2 blocks.

In fact, when cutting 2 knives, the two straight lines intersect, when cutting 3 knives, the third knife intersects the first two straight lines, and when cutting 4 knives, the fourth knife intersects with the first three straight lines ,..

And so on.

I looked at it all and felt that the question itself was not clearly explained. Only a scimitar or something, there is a little possibility. As shown in the figure, the red line is 3 cuts, and the result can be 9 pieces, but the premise must be that the knife has this arc.

If the knife can only be straight, then the problem is really to draw three straight lines in a circle, intersecting as much as possible. But even if two intersect in pairs, what is the result? It turned out that there were only 7 pieces of watermelon.

Since you can curve, why not use the S line, you can cut dozens of pieces.