Hiểu như này:

\(\dfrac{a}{1+a}+\dfrac{b}{1+b}+\dfrac{b}{1+b}=3-\left(\dfrac{1}{1+a}+\dfrac{1}{1+b}+\dfrac{1}{1+b}\right)\le3-\dfrac{9}{1+a+1+b+1+b}=\dfrac{3\left(a+2b\right)}{3+a+2b}\)

- Sự tiến triển của công cụ lao động: công cụ ghè đẽo (rìu cầm tay), công cụ rìu mài lưỡi; rìu có tra cán, cung tên…

- Sự tiến triển của cách thức lao động: săn bắt hái lượm, trồng trọt chăn nuôi.

V Transformation

1 In spite being quite poor, the villagers live a happy and healthy way

2 Despite studying very hard, he still didn't pass the exam

3 Although Sylvia had no interest in folklore, she still enjoyed the story

4 Despite having much experience in machinery, he didn't succeed in repairing this machine

5 Though it was dark, they continued to work

6 In spite of having health problems, he is always smiling

7 Despite the difficult exam, Kieu Anh got good marks

8 THe man about whom I told you works in the hospital

9 Do you know the girl to whom Tom is talking?

10 The tree which stankds near the gate of my house has lovely flowers

11 The book which I was reading yesterday was a lovely story

12 Do you know the new student whose name I can't remember?

13 I will never forget the day when I first met her

14 The country where I was born is beautiful

15 Because of the polluted water, it was unsafe to drink

16 Because he work hard and methodically, John succeeded in his exam

17 I suggest getting together and talking about our presentation before we do it in class

18 When did you built this stilt house?

19 If you don't hurry up, you will be late for school

20 You must turn off the TV before 11p.m

a: Xét ΔBAD có BA=BD

nên ΔBAD cân tại B

hay \(\widehat{BAD}=\widehat{BDA}\)

b: \(\widehat{HAD}+\widehat{BDA}=90^0\)

\(\widehat{CAD}+\widehat{BAD}=90^0\)

mà \(\widehat{BAD}=\widehat{BDA}\)

 nên \(\widehat{HAD}=\widehat{CAD}\)

hay AD là tia phân giác của góc HAC

c: Xét ΔADH vuông tại H và ΔADK vuông tại K có

AD chung

\(\widehat{HAD}=\widehat{KAD}\)

Do đó:ΔADH=ΔADK

Suy ra: AH=AK

 ( CH₃Br)

( C₂H₆ )

( C₂H₅Br )

a)

b) Vì đây là mạch điện gồm 2 đèn mắc nối tiếp nên:

U=U1+U2=2,4+2,5=4,9(V)

c) Vì đây là mạch điện gồm 2 đèn mắc nối tiếp nên:\

U=U1+U2 => U2 = U-U1=11,2-5,8=5,4(V)

d)

Vì đây là mạch điện gồm 2 đèn mắc nối tiếp nên:\

U=U1+U2 => U1 = U-U2=23,2-11,5=11,7(V)