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2010-03-22T15:38:25+01:00
V=s/t
s=v*t
s=120km

1h---60min
x----54 min
x=0,9h

v*t=120
(v+30)(t-0,9)=120

v=120/t
v*t-0,9v+30t-27=120

v=120/t
120-0,9*120/t+30t-27=120

v=120/t
-108/t+30t-27=120-120


v=120/t
-108/t+30t-27=0|*t

v=120/t
30t²-27t-108=0

30t²-27t-108=0
Δ=(-27)²-4*30*(-108)=729+12960=13689
√Δ=117
D:t>0
t₁=27+117/2*30=2,4∈D
t₂=27-117/2*30=-1,5∉D

v=120/2,4
v=50km/h
t=2,4 h
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