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Пояснение.
Решение.
\[\textbf{а)}\ \text{mn} - \text{mk} + \text{xk} - \text{xn} =\]
\[= \left( \text{mn} - \text{mk} \right) - \left( \text{xn} - \text{xk} \right) =\]
\[= m(n - k) - x(n - k) =\]
\[= (n - k)(m - x)\]
\[\textbf{б)}\ x^{2} + 7x - ax - 7a =\]
\[= \left( x^{2} + 7x \right) - (ax + 7a) =\]
\[= x(x + 7) - a(x + 7) =\]
\[= (x + 7)(x - a)\]
\[\textbf{в)}\ 3m - mk + 3k - k^{2} =\]
\[= (3m - mk) + \left( 3k - k^{2} \right) =\]
\[= m(3 - k) + k(3 - k) =\]
\[= (3 - k)(m + k)\]
\[\textbf{г)}\ xk - xy - x^{2} + yk =\]
\[= \left( - xy - x^{2} \right) + (xk + yk) =\]
\[= - x(y + x) + k(x + y) =\]
\[= (x + y)(k - x)\]