Giúp mk với ạ ☺️
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) ĐKXĐ:
x³ - 1 khác 0
x khác 1
b) A = (5x² + 5x + 5)/(x³ - 1)
= 5(x² + x + 1)/[(x - 1)(x² + x + 1)]
= 5/(x - 1)
Thay x = 7 vào A, ta được:
A = 5/(7 - 1)
= 5/6
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1 send - will receive
2 do - will improve
3 find - will give
4 will go - has
5 will go- gets
6 doesn't phone - will leave
7 don't study - won't pass
8 rains - won't have to
9 won't be able - watch
10 can't move - isn't
11 study - will pass
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1 me
2 too
3 done
doing
having
4 interested
5 as to
6 standing
when
7 helping
8 too
9 such
a
10 used
11 when
12 shouldn't
13 is - cost
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Áp dụng tính chất của dãy tỉ số bằng nhau, ta được
\(\dfrac{a}{3}=\dfrac{b}{4}=\dfrac{b-a}{4-3}=23\)
Do đó: a=69; b=92
![](https://rs.olm.vn/images/avt/0.png?1311)
Chào em, em tham khảo nhé!
1. won't finish
2. had learnt
3. is
4. didn't rain
5. had gone
6. hadn't told
7. would understand
8. will throw
9. would be
10. will post
11. will go
12. is hitting
13. won't do
14. will make
15. will go
16. go
17. eat
18. was washed
19. will be built
20. was cleaned
Chúc em học tốt và có những trải nghiệm tuyệt vời tại hoc24.vn!
![](https://rs.olm.vn/images/avt/0.png?1311)
1 C => hers
2 B => will use
3 D => their
4 A => will be pedaling
5 A => travel
6 C => less
7 D => have they
8 D => than
9 A => worked
10 B => fewer
![](https://rs.olm.vn/images/avt/0.png?1311)
Khi k mở, dòng điện chỉ chạy qua R1
Ta có: \(I=\dfrac{\varepsilon}{R+r}\Leftrightarrow2=\dfrac{6}{2+r}\Rightarrow r=1\Omega\)
Khi k đóng, mạch điện có sơ đồ: R1//R
Ta có, công suất tiêu thu trên R là: \(P=UI=I^2R\) (*)
Lại có: \(I=\dfrac{\varepsilon}{r+\dfrac{RR_1}{R+R_1}}\)
Thay I vào (*) ta có: \(P=\left(\dfrac{\varepsilon}{r+\dfrac{RR_1}{R+R_1}}\right)^2R=\dfrac{\varepsilon^2}{\left(\dfrac{r}{\sqrt{R}}+\dfrac{RR_1}{\sqrt{R}\left(R+R_1\right)}\right)}\)
\(P=\dfrac{\varepsilon^2}{\left(\dfrac{r}{\sqrt{R}}+\dfrac{RR_1}{\sqrt{R}\left(R+R_1\right)}\right)}=\dfrac{\varepsilon^2}{\left(\dfrac{r}{\sqrt{R}}+\dfrac{R_1\sqrt{R}}{\left(R+R_1\right)}\right)}\)
Để P cực đại thì mẫu số của nó phải đạt GTNN
Áp dụng bất đẳng thức Cosi vào 2 số \(\dfrac{r}{\sqrt{R}}\) và \(\dfrac{\sqrt{R}R_1}{\left(R+R_1\right)}\), ta có:
\(\dfrac{r}{\sqrt{R}}+\dfrac{\sqrt{R}R_1}{\left(R+R_1\right)}\ge2\sqrt{\dfrac{rR_1}{\left(R+R_1\right)}}\)
\(\dfrac{r}{\sqrt{R}}+\dfrac{R_1\sqrt{R}}{\left(R+R_1\right)}\) đạt giá trị nhỏ nhất khi \(\dfrac{r}{\sqrt{R}}=\dfrac{\sqrt{R}R_1}{\left(R+R_1\right)}\)
\(\Rightarrow r\left(R+R_1\right)=RR_1\Leftrightarrow1\left(R+2\right)=2R\Rightarrow R=2\Omega\)
Khi đó P=18W