ta có

t₁=\(\frac{s}{v_1}\) t₂=\(\frac{s}{v_2}\) t₃=\(\frac{s}{v_3}\)

Nên: v_tb=\(\frac{s+s+s}{t_1+t_2+t_3}\)=\(\frac{3s}{\frac{s}{v_1}+\frac{s}{v_2}+\frac{s}{v_3}}\)=\(\frac{3s}{s\left(\frac{1}{v_1}+\frac{1}{v_2}+\frac{1}{v_3}\right)}\)=\(\frac{3}{\frac{1}{v_1}+\frac{1}{v_2}+\frac{1}{v_3}}\)

a

1:C 2:D 3:D 4:B 5:C 6:B 7:C 8:A 9:D 10:D

nhiều quá vậy