Có Vchóp = 1/3 Sđáy.h = 1/3.a².2a =2/3a³

Có V. SAMN=V.SABD . 1/4=1/8V.SABCD=>.V.ABCDMN=7/8V.SABCD

=> V.ABCDMN = 7/8.2/3a³ = 7/12a³

7/12a³ nha

xy = \(\sqrt{x+r72y6}\)

Chắc để là tìm max

\(A=\sqrt{xy+3yz+2z^2}+\sqrt{yz+3xz+2x^2}+\sqrt{xz+3xy+2y^2}\)

Với x,y > 0 ta luôn có \(\sqrt{ab}\le\frac{a+b}{2}\)

Dấu "=" xảy ra khi a = b 

Áp dụng ta được: 

\(2\sqrt{\frac{3}{2}}\sqrt{xy+3yz+2z^2}\le\frac{3}{2}+xy+3yz+2z^2\)

Tương tự: \(2\sqrt{\frac{3}{2}}\sqrt{yz+3xz+2x^2}\le\frac{3}{2}+yz+3xz+2x^2\)

\(2\sqrt{\frac{3}{2}}\sqrt{xz+3xy+2y^2}\le\frac{3}{2}+xz+3xy+2y^2\)

Cộng theo vế ta được : 

\(2\sqrt{\frac{3}{2}}A\le\frac{9}{2}+4xy+4yz+4xz+2x^2+2y^2+2z^2\)

Ngoài ra với mọi số thực x,y,z  ta có : 

           \(x^2+y^2+z^2\ge xy+yz+xz\)

Dấu "=" xảy ra khi x = y = z 

\(\Rightarrow2\sqrt{\frac{3}{2}}A\le\frac{9}{2}+6\left(x^2+y^2+z^2\right)\le\frac{9}{2}+6\times\frac{3}{4}=9\)

\(\Rightarrow A\le\frac{3\sqrt{6}}{2}\).

Dấu "=" xảy ra khi \(x=y=z=\frac{1}{2}\)

tự làm

chúc học tốt

Tự làmđi 

Học tốt nhé

ko chép mạng thì thoy

= 100000

đề bài này sai rồi ! 

mình không biết mình lớp2

ko bết

 V. Read the text and choose the best answer to fill in the blank. There are two words that are not necessary.

paper -glass- old -metal -symbol-plass- cans -furniture -clear- televisions

We can recycle most things - clothes, shoes, phones, computers, (1) __televisions__________, cars, (2) ____plass________, papers, bottles, and batteries. There are a few things that we can’t recycle - but not many!We can recycle most (3) __paper____________ and card to make new paper. We can recycle (4) ___metal_________ from many different things cans, cars and computers. It’s hard to get new metal so it’s good to recycle metal.We can recycle most types of (5) __glass__________ It’s good to sort the glass into different colours - green, brown and clear glass. Factories can only make new clear glass when 99% of the recycle glass is (6)  ___clear_________There’s a recycling (7) ____symbol________ on a lot of the things that we can recycle. There’s sometimes a symbol on the things that we can’t recycle.We have to recycle different types of (8) __furniture__________ in different ways. The recycling symbols on plas help people to sort the different types of plas.

VI. Rearrange the words to make meaningful sentences.

1. How/ Julia/ often/ piano/ the/ does/ play?  How often does Julia play the piano ?

2. What/ Helen/ his/ sister/ do/ and/ do/ it’s/ hot/ when?  What do Helen and his sister do when it's hot ?

3. Family/ our/ going/ is/ for/ stay/ Nha Trang/ to/ beach/ 5 days/ in.  Our family is going to stay in Nha Trang beach for 5 days

4. Don’t/ why/ do/ homework/ before/ you/ out/ going/ your? Why don't you do your homework before going out ?

5. They/ playing/ with/ soccer/ are/ in/ park/ now/ the/ their/ friends.   They are playing soccer in the park with their friends now

\(\frac{1}{10}\)nha

để viết được dưới dạng só thập phân hữu hạn 

khi mẫu của phân số tối giản chỉ có ước nguyên tố là 2 và 5

vậy \(P=\frac{x}{3.5.x}\) có dạng số thập phân hữu hạn thì \(\hept{\begin{cases}x⋮3\\y\in\left\{2,5\right\}\end{cases}\Rightarrow\hept{\begin{cases}x=3\\y=2\text{ hoặc }y=5\end{cases}}}\)

b. ta có :\(Q=\frac{15x}{2.7y}\Rightarrow\hept{\begin{cases}x⋮7\\y\in\left\{2,5\right\}\end{cases}\Leftrightarrow\hept{\begin{cases}x=7\\y=2\text{ hoặc y=5}\end{cases}}}\)

Ta có:

Góc BOD + góc DOC = 1200

=> góc DOC = 1200 - góc BOD = 120o - 90o = 30o

Góc AOC + góc COB = 120o

=> góc COB = 120o - góc AOC= 120o - 90o = 300

mà Góc BOC + góc COD + góc DOA = 120o 

=> góc COD  = 120o - ( góc BOC + góc DOA) = 1200 - 600 = 600 

Ta có: 

Góc BOC = Góc AOD 

=> 1/2 BOC = 1/2 AOD = 30/2 = 5⁰
hay góc nOC = góc mOD = 15o

mà góc nOm= góc nOC +góc mOD + góc COD = 15o +150 +600 = 90o

hay nO vuông góc với mO.

k cho mình nha

OD vuông góc với OD ?????