\(S_{1_{ }}=v_1.t_1\Rightarrow t_1=\frac{S_1}{v_1}=\frac{S}{75}\)

\(S_2=v_2.t_3\Rightarrow t_2=\frac{S_2}{v_2}=\frac{2S}{3.v_2}\)

\(v_{tb}=\frac{S}{t}=\frac{S}{t_1+t_2}=\frac{20km}{h}\)

\(\Leftrightarrow225v_2=60v_2+300\Rightarrow v_2=18,\frac{18km}{h}\)

Giải:

Theo bài ra ta có  S 1 = v 1 . t 1 ⇒ t 1 = S 1 v 1 = S 75

S 2 = v 2 . t 2 ⇒ t 2 = S 2 v 2 = 2 S 3 v 2

Theo bài ra ta có  v t b = S t = S t 1 + t 2 = 20 k m / h ⇒ S S 75 + 2 S 3 v 2 = 20 k m / h

⇒ 225 v 2 = 60 v 2 + 3000 ⇒ v 2 = 18 , 182 k m / h

Tham khảo ạ

16km/h

Gọi nửa QĐ là S

vtb = 2s/(s/v1+s/v2) = 2/(1/12+1/20) = 15km/h

\(=>vtb=\dfrac{S}{\dfrac{\dfrac{1}{2}S}{v1}+\dfrac{\dfrac{1}{2}S}{v2}}=\dfrac{S}{\dfrac{S}{40}+\dfrac{S}{2v2}}=\dfrac{S}{\dfrac{S\left(2v2+40\right)}{80v2}}=\dfrac{80v2}{2v2+40}=15\)

\(=>v2=12km/h\)

\(v_{tb}=\dfrac{1}{\dfrac{\dfrac{1}{3}}{20}+\dfrac{\dfrac{2}{3}}{v_2}}=30\left(\dfrac{km}{h}\right)\\ \Leftrightarrow\dfrac{1}{\dfrac{1}{60}+\dfrac{2}{3v_2}}=30\\ \Leftrightarrow\dfrac{1}{2}+\dfrac{20}{v_2}=1\\ \Leftrightarrow\dfrac{20}{v_2}=\dfrac{1}{2}\\ Vậy:v_2=40\left(\dfrac{km}{h}\right)\)

Tham khảo nhé!

https://tailieumoi.vn/bai-viet/98187/mot-nguoi-di-xe-dap-tren-23-doan-duong-dau-voi-van-toc-trung-binh-10-kmh-kkljm