We use the observed jet boundary transition from parabolic to conical shape, which was earlier discovered as possibly a common effect in active galactic nuclei, to estimate a black hole, a jet and an ambient medium parameters. We explained earlier the geometry transition as a consequence of a change in the jet inner properties: a transition from a magnetically dominated to an equipartition regime. This interpretation allows us to estimate a black hole spin, a black hole mass and an ambient pressure amplitude, using the observed jet shape break position and the jet width at the transition point for 11 active galactic nuclei. The black hole spin values obtained using our method are consistent with the lower estimates for the sources with redshift $z<2$ from the spin evolution modelling. We find that the method of a black hole mass determination based on the relation between the broad-line region size and its luminosity may underestimate masses of the sources with large jet viewing angles. We propose a new method for the black hole mass determination, with the obtained masses being in interval $10^8-10^{10};M_{odot}$. The range of the values of the ambient pressure amplitude points to the uniform medium conditions for the sources in our sample, with a tentative indication of higher pressure around FRII sources.
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