helpppp

Gọi O là giao điểm của AC và BD. Trong mặt phẳng (SAC) kẻ OO1 vuông góc với SC, dễ thấy mp(BO1D) vuông góc với SC. Vậy góc giữa hai mp(SBC) và (SDC) bằng góc giữa hai đường thẳng BO1 và DO1. Mặt khác OO1 ⊥ BD, OO1 < OC mà OC = OB nên 

Tương tự  tức 

Như vậy hai mặt phẳng (SBC) và (SDC) tạo với nhau góc  khi và chỉ khi: 

   (vì ΔBO1D cân tại O1)
 

Ta lại có : 

Như vậy : 

Vậy khi x = a thì hai mặt phẳng (SBC) và (SDC) tạo với nhau góc 60˚

helppp ghi chi tiết dùm mình với ạ

Choose the word whose main stress pattern is placed differently from the other.

1. A. natural          B. disaster             C. volcano            D. typhoon

2. A. decorate        B. nominate          C. describe            D. celebrate

3. A. sewage         B. pollution          C. prevent             D. provide

4. A. plumber        B. label                 C. innovation        D. nuclear

   5. A. Acquaintance    B. Dynamite           C. Appliance          D. Pollution

   6. A. Compose           B. Pollute             C. Cover                   D. Install

Helppp
Chọn từ có phần gạch chân được phát âm khác với những từ còn lại trong mỗi câu sau.

1. A. garbage        B. standard           C. solar                 D. lunar

2. A. raised           B. picked              C. laughed            D. watched

3. A. energy          B. generous           C. category           D. suggest

4. A. celebrate       B. plumber            C. bulb                  D. blanket

9. A. nuclear        B. plumber           C. consumer         D. reduce

10. A. trick            B. tidal                  C. pipe                  D. climate

11. A. about          B. should              C. shout                D. mouse

12. A. activity       B. accept               C. across               D. afraid

13. A. move          B. hotel                 C. photo                D. pagoda

14. A. border         B. cyclone             C. destroy             D. forecast

15. A. earthquake    B.blanket                   C. sticky              D. know

16.A. worked            B. watched                C. cleaned            D. helped

17. A. looks              B. cats                       C. roofs                D. opens

18. A. eleven            B. employ                C. get                   D. complete

19. A. plumber         B. brother                 C. September       D. bill

20. A. flow                B. row                        C. know           D. now

21. A. fly                    B. hobby                    C. energy           D. ordinary

22. A. dump              B. bulb                       C. nuclear        D. plumber

23. A scheme            B. mechanic              C. machine       D. chemistry

24. A. tower             B. slow                      C. flower         D. down

25. A. Junkyard       B. Plumber               C. Adult                     D. Reduce

26. A. Scheme         B. Charity                 C. Chopstick               D. Check

Chọn từ có phần gạch chân được phát âm khác với những từ còn lại trong mỗi câu sau.

1. A. garbage        B. standard           C. solar                 D. lunar

2. A. raised           B. picked              C. laughed            D. watched

3. A. energy          B. generous           C. category           D. suggest

4. A. celebrate       B. plumber            C. bulb                  D. blanket

9. A. nuclear        B. plumber           C. consumer         D. reduce

10. A. trick            B. tidal                  C. pipe                  D. climate

11. A. about          B. should              C. shout                D. mouse

12. A. activity       B. accept               C. across               D. afraid

13. A. move          B. hotel                 C. photo                D. pagoda

14. A. border         B. cyclone             C. destroy             D. forecast

15. A. earthquake    B.blanket                   C. sticky              D. know

16.A. worked            B. watched                C. cleaned            D. helped

17. A. looks              B. cats                       C. roofs                D. opens

18. A. eleven            B. employ                C. get                   D. complete

19. A. plumber         B. brother                 C. September       D. bill

20. A. flow                B. row                        C. know           D. now

21. A. fly                    B. hobby                    C. energy           D. ordinary

22. A. dump              B. bulb                       C. nuclear        D. plumber

23. A scheme            B. mechanic              C. machine       D. chemistry

24. A. tower             B. slow                      C. flower         D. down

25. A. Junkyard       B. Plumber               C. Adult                     D. Reduce

26. A. Scheme         B. Charity                 C. Chopstick               D. Check

Environment             population                build               different         providing                   result

DEFORESTATION

Deforestation means cutting down forests that are home (1)___ ____ human beings with oxygen. Forests around the world are being cut down for (2)_ different _ purposes. One of the main aims is to use the land for agriculture. With the growing (3)____________________ there is a need to grow more crops to satisfy people's demand. Forests are also being cut to (4) ____________________ towns and cities for setting up offices and factories. As a (5)_________________ , not only is the weather becoming hotter and hotter, but many species of animals and plants are getting extinct because of deforestation as well. We should consider the harmful impacts of deforestation on our (6)_________________ and put an end to it.

Environment             population                build               different         providing                   result

DEFORESTATION

Deforestation means cutting down forests that are home (1)___ ____ human beings with oxygen. Forests around the world are being cut down for (2)_ different _ purposes. One of the main aims is to use the land for agriculture. With the growing (3)____________________ there is a need to grow more crops to satisfy people's demand. Forests are also being cut to (4) ____________________ towns and cities for setting up offices and factories. As a (5)_________________ , not only is the weather becoming hotter and hotter, but many species of animals and plants are getting extinct because of deforestation as well. We should consider the harmful impacts of deforestation on our (6)_____________       ____ and put an end to it.