Đổi:

15' = 0,25h

30' = 0,5h.

Gọi t₁ là thời gian hai xe gặp nhau khi đi ngược chiều; t₂ là thời gian hai xe gặp nhau khi đi cùng chiều.

Tổng vận tốc của hai xe là:

\(t_1=\dfrac{S}{v_1+v_2}=\dfrac{20}{v_1+v_2}=0,25\)

\(\Rightarrow v_1+v_2=80.\)

Hiệu vận tốc của hai xe là:

\(t_2=\dfrac{S}{v_1-v_2}=\dfrac{20}{v_1-v_2}=0,5\)

\(\Rightarrow v_1-v_2=40.\)

Vận tốc của xe thứ nhất là:

\(v_1=\dfrac{80+40}{2}=60\left(\dfrac{km}{h}\right)\)

Vận tốc củ xe thứ hai là:

\(v_2=\dfrac{80-40}{2}=20\left(\dfrac{km}{h}\right)\)

Nguyễn Đình Phương trả lời đúng 1l ike

1.The students mustn't (stay) stay up too late

2.Let's (go) go somewhere for a drink

3.Let's (sing) sing some english songs

4.What about (dance) dancing?

5.How about (play) playing soccer?

6.Why don't we(go) go out for a wwalk?

7.The weather (not be) is not warm today

8.What (be) is the wweather like in the winter?

-It (be) is very cold

9. Do you always(camp) camp in the summer?

10. Does your uncle(plant) plant many flowers in the garden?

11 What does your father often(do) do in the evening?

-he often (wath) watches televison ,but sometimes he (read) reads books

12.Hoa's brother (be) is an engineer,but he (not work) isn't working at the moment

13.We (not go) don't go to the zoo very often

14.Nam usually (play) plays sports,but this afternoon he(fly) is flying his kite