130. A triangular pyramid $S A B C$ is inscribed in a cone ( $S$ coincides with the vertex of the cone, $A, B$ and $C$ lie on the circumference of the base of the cone), the dihedral angles at the edges $S A, S B$ and $S C$ are equal to $\alpha, \beta$ and $\gamma$ respectively. Find the angle between the plane $S B C$ and the plane tangent to the surface of the cone along the generatrix $S C$.
130. Let's draw $S O$ - the height of the pyramid. This creates three pyramids $S A B O, S B C O, S C A O$. In each of these pyramids, the dihedral angles at the lateral edges $S A$ and $S B, S B$ and $S C, S C$ and $S A$ are right angles. Denote these angles by $x, y$, and $z$. We obtain the system

$$
\left\{\begin{array}{l}
x+y=\beta \\
y+z=\gamma \\
z+x=\alpha
\end{array}\right.
$$

from which we find $z=\frac{\alpha-\beta+\gamma}{2}$, and the desired angle will be $\frac{\pi-\alpha+\beta-\gamma}{2}$.