  A measurement is presented of the ratio of the inclusive 3-jet cross section
to the inclusive 2-jet cross section as a function of the average transverse
momentum, <pT[1,2]>, of the two leading jets in the event. The data sample was
collected during 2011 at a proton-proton centre-of-mass energy of 7 TeV with
the CMS detector at the LHC, corresponding to an integrated luminosity of 5.0
inverse femtobarns. The strong coupling constant at the scale of the Z boson
mass is determined to be alphaS[MZ] = 0.1148 +/- 0.0014 (exp.) +/- 0.0018 (PDF)
+/- 0.0050 (theory), by comparing the ratio in the range 0.42 < <pT[1,2]> <
1.39 TeV to the predictions of perturbative QCD at next-to-leading order. This
is the first determination of alphaS[MZ] from measurements at momentum scales
beyond 0.6 TeV. The predicted ratio depends only indirectly on the evolution of
the parton distribution functions of the proton such that this measurement also
serves as a test of the evolution of the strong coupling constant. No deviation
from the expected behaviour is observed.
