  A simple geometrical model with event-by-event fluctuations is suggested to
study elliptical and triangular eccentricities in the initial state of
relativistic heavy-ion collisions. This model describes rather well the ALICE
and ATLAS data for Pb+Pb collisions at center-of-mass energy $\sqrt{s_{NN}} =
5.02$~TeV per nucleon pair, assuming that the second, $v_2$, and third, $v_3$,
harmonics of the anisotropic flow are simply linearly proportional to the
eccentricities $\varepsilon_2$ and $\varepsilon_3$, respectively. We show that
the eccentricity $\varepsilon_3$ has a pure fluctuation origin and is
substantially dependent on the size of the overlap area only, while the
eccentricity $\varepsilon_2$ is mainly related to the average collision
geometry. Elliptic flow, therefore, is weakly dependent on the event-by-event
fluctuations everywhere except of the very central collisions 0--2%, whereas
triangular flow is mostly determined by the fluctuations. The scaling
dependence of the magnitude of the flow harmonics on atomic number, $v_n
\propto A^{-1/3}$, is predicted for this centrality interval.
