If the number of edges of a polyhedron is 8, then the number of faces of the polyhedron is 8____ ； If the number of faces of a geometry is f, the number of vertices is V, and the number of edges is e, the value of F + V-E of the geometry is______ .

If the number of edges of a polyhedron is 8, then the number of faces of the polyhedron is 8____ ； If the number of faces of a geometry is f, the number of vertices is V, and the number of edges is e, the value of F + V-E of the geometry is______ .

If the number of edges is 8, it can be a pentahedron with 5 vertices. The formula is F + V-E = 2, which is 5 + 5-8 = 2. Perfect!

What is the base of a space regular tetrahedron? What is the projection of the vertex on the base? What is the center of a triangle

Each side can be the bottom
When the three sides are equal, the projection of the vertex on the bottom is the outer center of the triangle
When the angles of the three sides and the bottom are equal, the projection of the vertex on the bottom is the inner part of the triangle
When the opposite edges of a regular tetrahedron are perpendicular to each other, the projection of the vertex on the bottom is the perpendicular of the triangle

If we say that the projection of the vertex of a triangular pyramid on the bottom is the perpendicular of the triangle, can we deduce that the three sides are perpendicular to each other?

No, to the contrary
If this proposition is correct, then find a counterexample
Regular triangular pyramid

The bottom surface ABC of the triangular pyramid d-abc is an acute triangle, and DA is perpendicular to the plane ABC, h is the projection of a in the plane BCD

By using the method of disprovement, it is assumed that h is the perpendicular of △ BCD
Extend BH to Cd in E
∵ h is the projection of a in plane BCD, ∵ ah ⊥ plane BCD, ∵ CD ⊥ ah
It is assumed that h is the perpendicular of △ BCD, CD ⊥ BH
From CD ⊥ ah, CD ⊥ BH, ah ∩ BH = h, we get: CD ⊥ plane ABH, ∩ ab ⊥ CD
∵ ad ⊥ plane ABC, ∵ ab ⊥ ad
From ab ⊥ CD, ab ⊥ ad, ab ∩ ad = a, we get: ab ⊥ plane ACD, ∩ ab ⊥ AC
This is contrary to the fact that △ ABC is an acute triangle
It is impossible that h is perpendicular to △ BCD

How to do the mechanical efficiency in the ninth grade of junior high school physics innovation guide manual

First of all, it requires the active work and the total work. The active work is generally in the ninth grade topic, which is the product of gravity and the lifting height of heavy objects. The total work is equal to the pulling force times the lifting height of the rope, that is, w you = G times h, w total = FS. Mechanical efficiency = w you divided by W total times 100%, which is the mechanical efficiency
That's it! O (∩)_ ∩)o

Question 9 P9 question 12

It's easy to answer the question directly

P77-79, the guide manual for Chinese innovation of Jiangsu education press grade 7 Volume 1, I didn't bring it home,

Practice after class in Lesson 8 of the seventh grade Chinese volume I of Jiangsu Education Press. The first volume of Junior High School of Jiangsu Education Press. Daring to innovate is the key period to tap the potential of life and develop the wisdom of life

The answer to the second lesson of "miss in autumn" in Grade 7 of the new homework of Chinese guidance in 2013

1. The reasons why this article repeatedly wrote the content of "seeing flowers" are as follows:__________________________ .
[answer]: ① "looking at flowers" is the central event of the full text. ② "looking at flowers" is the concentrated embodiment of maternal love. ③ "looking at flowers" is the symbol of my new life and the achievement of my mother's wishes
2. The word "quietly" added in the article emphasizes that:____________________________ .
[answer]: ① the mother is very considerate of her son's difficulties. ② the mother is considerate and careful, and does not disturb her son as much as possible. ③ the mother's heart is even worse
3. There are several parts in the text. Please find one and mark it with a horizontal line, and then talk about its specific role in the text
A:________________________________________________________
Answer: First: "mother likes flowers..." Mother is a person who loves life very much. Mother gives up the enjoyment of life for her son
Second: "later my sister told me that my mother often had liver pain..." ① Secondly, the extraordinary efforts made by the mother
Third: "people tell me..." It shows that the mother only thinks about the happiness of her children and never cares about herself
4. Combined with the content, why the verbs "Pu" and "ban" are used well
A:__________________________________________________________
Answer: the mother always thinks about her son, and the detailed description makes the character vivid and touching
5. In the third paragraph, would you like to replace the "request" with "expectation"? Why?
A:__________________________________________________________
[answer]: ① "begging" reflects the complex psychology of the mother, who is not only looking forward to her son getting rid of the shadow, but also afraid that she will not be able to support that day. If it is replaced by "expecting", it can not highlight the feature that the mother is still thinking for her son despite the great pain. ② from the meaning of the word, "begging" means minimum requirement, while "expecting" means higher requirement
6. The end of the article says "I understand what my mother hasn't finished." do you think the "I" in the article understands? Please explain it with the description in the ending paragraph
A:__________________________________________________________
[answer]: from the description at the end, "I" not only understood, but also no longer depressed as my mother expected. I supported each other with "sister" and actively faced life
7. If the word "autumn" is removed from the title of this article, why not?
A:__________________________________________________________
[answer]: ① "autumn" is the background and the introduction. The scenery, narration and people in the article are all related to "autumn". ② "autumn" is the portrayal of "I"'s unfortunate experience and gloomy mood, which is symbolic. ③ "autumn" is the symbol of "I" coming out of the trough and optimistic life

Mathematics is the edition of Beijing Normal University, chemistry is the edition of science and education, others are the edition of Shandong education
In addition, the fourth day of the total review, what to buy exercise books?

"Five year high school entrance examination three-year simulation" is good, there are root textbooks, synchronous also have high school entrance examination review
And "Dian Dian Dian" and "double speed training method"

What are the good books or exercise books for English, mathematics, physics and chemistry?

The three-year high school entrance examination five-year simulation... I do this... The questions are all the high school entrance examination questions over the years, the most important thing is, if you do it at home, the answers attached to this teaching assistant are very detailed, you can understand the wrong questions by looking at the answers