Diện tích: \(\dfrac{8^2}{4}.3,14=50,24\left(cm^2\right)\)

Vận tốc khi thuyền đi ngược dòng nước:

   \(7,2-1,6=5,6\left(km/h\right)\)

C

2Ca + O₂ ---t^o→ 2CaO

4Na + O₂ ---t^o​→ 2Na₂O

S + O₂ ---t^o​→ SO₂

3Fe + 2O₂ ---t^o​→ Fe₃O₄

C + O₂ ---t^o​→ CO₂

4Na + O₂ ---t^o​→ 2Na₂O

2Cu + O₂ ---t^o​→ 2CuO

2Zn + O₂ ---t^o​→ 2ZnO

2Mg + O₂ ---t^o​→ 2MgO

CH₄ + 2O₂ ---t^o​→ CO₂ + 2H₂O

2Ba + O₂ ---t^o​→ 2BaO

em nhìn nhầm câu cuối 

it takes me six hours to go Vinh to Ha Noi

11. C

12. A

13. A

14. B

15. D

16. B

17. A

18. B

19. B

20. C

21. A

Ta có: \(m_1.v_1=\left(m_1+m_2\right).v\)

    \(\Leftrightarrow v=\dfrac{m_1v_1}{m_1+m_2}=\dfrac{50.10}{50+200}=2\left(m/s\right)\)

a,Ta có: \(\dfrac{AB}{AC}=0,75=\dfrac{3}{4}\Leftrightarrow\dfrac{AB^2}{AC^2}=\dfrac{9}{16}\)

 \(\Rightarrow\dfrac{AB^2}{9}=\dfrac{AC^2}{16}=\dfrac{AB^2+AC^2}{9+16}=\dfrac{BC^2}{25}=\dfrac{15^2}{25}=9\)

\(\Rightarrow AB^2=9.9=81\Leftrightarrow AB=9\left(cm\right);AC^2=9.16=144\Leftrightarrow AC=12\left(cm\right)\)

b, Ta có: \(S_{ABC}=\dfrac{1}{2}.AB.AC\)

Mà \(S_{ABC}=\dfrac{1}{2}.AH.BC\)

 \(\Rightarrow AB.AC=AH.BC\Leftrightarrow AH=\dfrac{AB.AC}{BC}=\dfrac{9.12}{15}=7,2\left(cm\right)\)

Ta có:\(DE=BD+BC+CE=AB+BC+AC\)