1. isn't

2. wasn't

3. doesn't

4. can't

5. will

6. didn't

7. hasn't

8. can't

9. did

10. does

11. couldn't

12. must

13. aren't

14. did

15. don't

16. won't

17. doesn't

18. isn't

19. was

20. don't

13. Tea can't be made with cold water.

14. Some of my books have been taken away by somebody.

15. The meeting will be held before May Day.

$AD$ là tia phân giác $\widehat A$

$\Rightarrow \dfrac{AB}{AC}=\dfrac{BD}{CD}$ hay $\dfrac{3}{4}=\dfrac{3}{CD}$

$\Leftrightarrow CD=\dfrac{3.4}{3}=4(cm)$

Ta có: $BD+CD=BC$

$\Rightarrow 3+4=BC$

$\Rightarrow 7cm=BC$

$\to b)$