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\[\textbf{а)}\ k = \frac{6}{12} = \frac{1}{2}.\]
\[\textbf{б)}\ \angle A_{1} = 90{^\circ};\]
\[\angle C_{1} = 100{^\circ}.\]
\[\textbf{в)}\ A_{1}B_{1} = 4 \cdot 2 = 8;\]
\[CD = 10\ :2 = 5.\]