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\[\textbf{а)}\ S = 16\ см^{2}:\ \]
\[a = \sqrt{16} = 4\ см.\]
\[\textbf{б)}\ S = 2,25\ дм^{2}:\ \]
\[a = \ \sqrt{2,25} = 1,5\ дм.\]
\[\textbf{в)}\ S = 12\ м^{2}:\]
\[a = \sqrt{12} = 2\sqrt{3}\ м.\]