Question

o barca parcurge o distanta d in sensul de curgere al unui rau in 20 de minute iar in sens opus in 40 de minute .Stiind ca barca parcurge pe un lac in 30 de minute o distanta de 9 km sa se afle viteza de curgere a raului

Answer 1

Se da:
t1=20min=1200s
t2=40min=2400s
t=30min=1800s
d=9km=9000m
v0=?m/s

Formule:
v=d/t

1)v+v0=v1
(d/t)+v0=d1/t1
d1=(t1*d)/t+t1*v0

2)v-v0=v2
(d/t)-v0=d1/t2
d1=(t2*d)/t-t2*v0

Din 1) si 2) 
(t1*d)/t+t1*v0=(t2*d)/t-t2*v0
t1*v0+t2*v0=(t2*d)/t-(t1*d)/t
v0(t1+t2)=d(t2-t1)/t
v0=d(t2-t1)/t(t1+t2)

Calcule:
v0=9000*(2400-1200)/1800*(1200+2400)=1,(6)m/s