HOC24
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t1 = \(\dfrac{S_1}{v_1}\)= \(\dfrac{S}{\dfrac{3}{v_1}}\) t = \(\dfrac{S_2}{v_2}\)=\(\dfrac{S}{\dfrac{3}{v_2}}\) t =\(\dfrac{S_3}{v_3}\)=\(\dfrac{S}{\dfrac{3}{v_3}}\) ⇒vtb = \(\dfrac{S}{\dfrac{S}{\dfrac{3}{v_1}}+\dfrac{S}{\dfrac{3}{v_2}}+\dfrac{S}{\dfrac{3}{v_3}}}\)=\(\dfrac{S}{\dfrac{S}{3v_1}+\dfrac{S}{3v_2}+\dfrac{S}{3v_3}}\) =\(\dfrac{S}{\text{S( }\dfrac{1}{v_1}+\dfrac{1}{v_2}+\dfrac{1}{v_3}}\) = \(\dfrac{3v_1v_2v_3}{v_2v_3+v_1v_3+v_1v_2}\) = \(\dfrac{3\cdot12\cdot15\cdot19}{15\cdot19+12\cdot19+12\cdot15}\)=\(\dfrac{10260}{84825}\)= ... km/h