12.26. On a plane, three angles equal to the face angles $\alpha, \beta$, and $\gamma$ of a trihedral angle are drawn. Construct on the same plane an angle equal to the dihedral angle opposite the face angle $\alpha$.
12.26. Consider a trihedral angle with plane angles $\alpha, \beta$, and $\gamma$. Let $O$ be its vertex. On the edge opposite to angle $\alpha$, take a point $A$ and draw perpendiculars $A B$ and $A C$ to the edge $O A$ in the planes of the faces. This construction can be performed on the given plane for the development of the trihedral angle (Fig. 94). Now, construct triangle $B A^{\prime} C$ with sides $B A^{\prime}=B A_{\mathbf{f}}$ and $C A^{\prime}=C A_{2}$. The angle $B A^{\prime} C$ is the desired one.

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Fig. 05 This construction can be carried out on the given plane for the development of the trihedral angle (Fig. 94). Now, construct triangle $B A^{\prime} C$ with sides $B A^{\prime}=B A_{\mathbf{f}}$ and $C A^{\prime}=C A_{2}$. The angle $B A^{\prime} C$ is the desired one.