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15 tháng 5 lúc 10:05

thank you teacher

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15 tháng 5 lúc 12:57

thanks teacher

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11 tháng 6 lúc 8:36

Bài 5

$a - b = 2 <=> b = a - 2$

Do đó: $P = 3a^2 + (a-2)^2 + 8$

$= 3a^2 + a^2 - 4a + 4 + 8$

$= 4a^2 - 4a + 12$

$= (2a - 1)^2 + 11$

Vì $(2a - 1)^2 \geq 0$ với mọi a nên $= (2a - 1)^2 + 11 \geq 11$ hay $P \geq 11$

Dấu "=" xảy ra $\begin{cases} a - b = 2 \\ 2a - 1 = 0 \\\end{cases} <=> \begin{cases} a = \dfrac{1}{2} \\ b = -\dfrac{3}{2} \\\end{cases}$

Vậy giá trị nhỏ nhất của P là 11 tại $\begin{cases} a = \dfrac{1}{2} \\ b = -\dfrac{3}{2} \\\end{cases}$

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11 tháng 6 lúc 11:12

câu hình:

a) Vì AB là đường kính $\Rightarrow\angle ADB=90\Rightarrow\angle EDB+\angle EHB=180$

$\Rightarrow EDHB$ nội tiếp

b) Xét $\Delta AHE$ và $\Delta ADB:$ Ta có: $\left\{{}\begin{matrix}\angle DABchung\\\angle AHE=\angle ADB=90\end{matrix}\right.$

$\Rightarrow\Delta AHE\sim\Delta ADB\left(g-g\right)\Rightarrow\dfrac{AH}{AD}=\dfrac{AE}{AB}\Rightarrow AB.AH=AD.AE$

mà $AH.AB=AC^2$ (hệ thức lượng) $\Rightarrow AC^2=AD.AE$

c) Vì $EF\parallel AB$ $\Rightarrow\angle CFE=\angle CBA=\angle CDA=\angle CDE$

$\Rightarrow CDFE$ nội tiếp mà $\angle CEF=90$ $(EF\parallel AB,AB\bot CH)$

$\Rightarrow\angle CDF=90\Rightarrow CD\bot DF$

Vì $\Delta CDF$ vuông tại D có K là trung điểm CF $\Rightarrow KC=KD$

$\Rightarrow\Delta KCD$ cân tại K $\Rightarrow\angle DKB=2\angle DCB=2\angle DAB=\angle DOB$

$\Rightarrow DKOB$ nội tiếp $\Rightarrow K\in\left(OBD\right)$

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11 tháng 6 lúc 10:03

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Hôm qua lúc 10:55

Câu 1

1) $A=\sqrt{20}-\sqrt{45}+\sqrt{6+2\sqrt{5}}$

$=2\sqrt{5}-3\sqrt{5}+\sqrt{\left(\sqrt{5}\right)^2+2\sqrt{5}+1}$

$=-\sqrt{5}+\sqrt{\left(\sqrt{5}+1\right)^2}$

$=-\sqrt{5}+\sqrt{5}+1$

$=1$

2) ĐKXĐ: $x>0;x\ne1$

$B=\left(\dfrac{1}{x-\sqrt{x}}+\dfrac{1}{\sqrt{x}-1}\right):\dfrac{\sqrt{x}+1}{\left(\sqrt{x}-1\right)^2}$

$=\left(\dfrac{1}{\sqrt{x}\left(\sqrt{x}-1\right)}+\dfrac{1}{\sqrt{x}-1}\right).\dfrac{\left(\sqrt{x}-1\right)^2}{\sqrt{x}+1}$

$=\dfrac{1+\sqrt{x}}{\sqrt{x}\left(\sqrt{x}-1\right)}.\dfrac{\left(\sqrt{x}-1\right)^2}{\sqrt{x}+1}$

$=\dfrac{\sqrt{x}-1}{\sqrt{x}}$

$\Rightarrow B=\dfrac{\sqrt{x}-1}{\sqrt{x}}=\dfrac{1}{2}$

$\Leftrightarrow2\left(\sqrt{x}-1\right)=\sqrt{x}$

$\Leftrightarrow2\sqrt{x}-2=\sqrt{x}$

$\Leftrightarrow2\sqrt{x}-\sqrt{x}=2$

$\Leftrightarrow\sqrt{x}=2$

$\Rightarrow x=4\left(tmđk\right)$

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Hôm qua lúc 11:01

Câu 5 :

Ta có : $6a+3b+2c=abc$ $\Rightarrow\dfrac{6}{bc}+\dfrac{3}{ac}+\dfrac{2}{ab}=1$

Đặt : $\left\{{}\begin{matrix}\dfrac{1}{a}=x\\\dfrac{2}{b}=y\\\dfrac{3}{c}=z\end{matrix}\right.$ với $x,y,z>0$ $\Rightarrow xy+yz+zx=1$

Ta biến đổi : $\dfrac{1}{\sqrt{a^2+1}}=\sqrt{\dfrac{\dfrac{1}{a^2}}{1+\dfrac{1}{a^2}}}=\sqrt{\dfrac{x^2}{1+x^2}}=\sqrt{\dfrac{x^2}{xy+xz+yz+x^2}}=\sqrt{\dfrac{x^2}{\left(x+y\right)\left(x+z\right)}}$

Chứng minh tương tự ta có :

$\dfrac{1}{\sqrt{b^2+4}}=\sqrt{\dfrac{y^2}{\left(y+z\right)\left(y+x\right)}}$ ; $\dfrac{1}{\sqrt{c^2+9}}=\sqrt{\dfrac{z^2}{\left(z+x\right)\left(z+y\right)}}$

Do đó : $Q=\sqrt{\dfrac{x^2}{\left(x+z\right)\left(x+y\right)}}+\sqrt{\dfrac{y^2}{\left(y+z\right)\left(y+x\right)}}+\sqrt{\dfrac{z^2}{\left(z+x\right)\left(z+y\right)}}$

Áp dụng BĐT Cô - si ta được :

$Q=\sqrt{\dfrac{x^2}{\left(x+z\right)\left(x+y\right)}}+\sqrt{\dfrac{y^2}{\left(y+z\right)\left(y+x\right)}}+\sqrt{\dfrac{z^2}{\left(z+x\right)\left(z+y\right)}}$

$\le\dfrac{1}{2}\left(\dfrac{x}{x+z}+\dfrac{x}{x+y}+\dfrac{y}{y+z}+\dfrac{y}{y+x}+\dfrac{z}{z+x}+\dfrac{z}{z+y}\right)$

$=\dfrac{1}{2}\left(\dfrac{x+z}{x+z}+\dfrac{x+y}{x+y}+\dfrac{z+y}{z+y}\right)=\dfrac{1}{2}\cdot3=\dfrac{3}{2}$

Dấu "=" xảy ra khi $\left\{{}\begin{matrix}a=\sqrt{3}\\b=2\sqrt{3}\\c=3\sqrt{3}\end{matrix}\right.$

Vậy Max $P=\dfrac{3}{2}$ khi $\left\{{}\begin{matrix}a=\sqrt{3}\\b=2\sqrt{3}\\c=3\sqrt{3}\end{matrix}\right.$

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Hôm qua lúc 11:08

Câu 2

1) Phương trình $x^2-6x+5=0$ có:

$a+b+c=1-6+5=0$

$\Rightarrow$ Phương trình có hai nghiệm phân biệt $x_1=1$ và $x_2=\dfrac{c}{a}=5$

2) $\left\{{}\begin{matrix}2\left(x+2\right)=3\left(y-1\right)\\3x+y=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+4=3y-3\\3x+y=6\end{matrix}\right.$

$\Leftrightarrow\left\{{}\begin{matrix}2x-3y=-7\\3x+y=6\end{matrix}\right.$

$\Leftrightarrow\left\{{}\begin{matrix}2x-3y=-7\\9x+3y=18\end{matrix}\right.$

$\Leftrightarrow\left\{{}\begin{matrix}11x=11\\3x+y=6\end{matrix}\right.$

$\Leftrightarrow\left\{{}\begin{matrix}x=1\\3+y=6\end{matrix}\right.$

$\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=3\end{matrix}\right.$

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12 tháng 5 lúc 9:10

thank you so much

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12 tháng 5 lúc 9:16

ra 1 bài anh đi cô lớp 7 đi cô

để cho em thử sức viết

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12 tháng 5 lúc 9:50

Cảm ơn cô:33

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15 tháng 4 lúc 0:00

1:  Only with careful environmental planning can we protect the world in which we live.

A.  Careful environmental planning protects the world we live in.

B.  Planning the environment carefully, we can protect the world in which we live.

C.  Protecting the world we live in, we plan the environment carefully.

D.  We can protect the world we live in only with careful environmental planning.

2:  You should take regular exercises instead of sitting in front of the television all day.

A.  Taking regular exercises is better than sitting in front of the television all day.

B.  Sitting in front of the television all day helps you take regular exercises.

C.  Sitting in front of the television all day and taking exercises are advisable.

D.  Don’t take regular exercises, just sit in front of the television all day.

3:  It was not until after I got home that I realized I had not set the burglar alarm in the office.

A.  On the way home, I suddenly realized that I had forgotten to turn on the burglar alarm in the office.

B.  Fortunately, I realized that I hadn't set the burglar alarm just before I left for home; otherwise, I would have had to travel all the way back to the office.

C.  I didn’t turn the burglar alarm on before I left the office, but I only became aware of this after I'd arrived home.

D. I wish I had realized before I arrived home that I hadn't turned on the burglar alarm in the office, then it would have been easier to go and set it.

4:  He did not remember the meeting. He went out for a coffee with his friends then.

A.  Not remember the meeting, he went out for a coffee with his friends.

B.  Not to remember the meeting, he went out for a coffee with his friends.

C.  Not remembered the meeting, he went out for a coffee with his friends.

D.  Not remembering the meeting, he went out for a coffee with his friends.

5:  They left their home early. They didn’t want to miss the first train.

A.  They left their home early so as to not miss the first train.

B.  They left their home early for fear that they wouldn’t miss the first train.

C.  They left their home early so as not to miss the first train.

D.  They left their home early in order that not to miss the first train.

6:  I’m sure Luisa was very disappointed when she failed the exam

A.  Luisa must be very disappointed when she failed the exam.

B.  Luisa must have been very disappointed when she failed the exam.

C. Luisa may be very disappointed when she failed the exam.

D. Luisa could have been very disappointed when she failed the exam.

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15 tháng 4 lúc 9:43

1:  Only with careful environmental planning can we protect the world in which we live.

D.  We can protect the world we live in only with careful environmental planning.

2:  You should take regular exercises instead of sitting in front of the television all day.

A.  Taking regular exercises is better than sitting in front of the television all day.

3:  It was not until after I got home that I realized I had not set the burglar alarm in the office.

C.  I didn’t turn the burglar alarm on before I left the office, but I only became aware of this after I'd arrived home.

4:  He did not remember the meeting. He went out for a coffee with his friends then.

D.  Not remembering the meeting, he went out for a coffee with his friends.

5:  They left their home early. They didn’t want to miss the first train.

C.  They left their home early so as not to miss the first train.

D.  They left their home early in order that not to miss the first train.

6:  I’m sure Luisa was very disappointed when she failed the exam

B.  Luisa must have been very disappointed when she failed the exam.

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11 tháng 5 lúc 13:53

thank you so much!!!!!!!!!!!!!!!!

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12 tháng 5 lúc 10:30

I would like to send this video to you, I just want to introduce to you this website, it is really great for those who want to learn English.We will learn skills from listening to reading. , write.From learning this, my English skills have improved a lot so I hope you can take some time to try this website.have a good day !!!!         :))))

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12 tháng 5 lúc 15:57

I. Change into passive.

1.  John collects money.

....................................................................................................................................

2.  Anna opened the window.

....................................................................................................................................

3.  We have done our homework.

....................................................................................................................................

4.  I will ask a question.

....................................................................................................................................

5.  He can cut out the picture.

....................................................................................................................................

6.  The sheep ate a lot.

....................................................................................................................................

7.  We do not clean our rooms.

....................................................................................................................................

8.  William will not repair the car.

....................................................................................................................................

9.  Did Sue draw this circle?

....................................................................................................................................

10. Could you feed the dog?

....................................................................................................................................

II: Rewrite these following sentences with the same meaning
1. She said, "I am reading."
→ She said that…………………………………………………………….
2. They said, "We are busy."
→ They said that ……………………………………………………………….
3. He said, "I know a better restaurant."
→ He said that………………………………………………………………...
4. She said, "I woke up early."
→ She said that……………………………………………………………….
5. He said, "I will ring her."
→ He said that………………………………………………………………….
6. They said, "We have just arrived."
→ They said that………………………………………………………………
7. He said, "I will clean the car."
→ He said that……………………………………………………………….
8. She said, "I did not say that."
→ She said that…………………………………………………………….
9. She said, "I don't know where my shoes are."
→ She said that……………………………………………………………….
10. He said: "I won't tell anyone."
→ He said that………………………………………………………………

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26 tháng 5 lúc 6:46

thanks you

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26 tháng 5 lúc 8:43

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2 giờ trước (19:56)

Đề tỉnh khác khó quá, không biết đề tỉnh e ra sao :v, đang chờ các e thi

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2 giờ trước (19:57)

đề sáng ni zừa thi xog nek

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2 giờ trước (20:14)

lạy trời câu cuối(:((()

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13 tháng 5 lúc 10:34

cảm ơn cô :333

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13 tháng 5 lúc 10:35

cảm ơn cô

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13 tháng 5 lúc 11:14

cảm ơn cô nha

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