Bài tập 31 trang 40 SGK Toán 7 Tập 2

\(\begin{array}{l} M + N = 3xyz - 3{{\rm{x}}^2} + 5{\rm{x}}y - 1 + 5{{\rm{x}}^2} + xyz - 5{\rm{x}}y + 3 - y\\ = 3{\rm{x}}yz + xyz - 3{{\rm{x}}^2} + 5{{\rm{x}}^2} + 5{\rm{x}}y - 5{\rm{x}}y - 1 + 3 - y\\ = 4{\rm{xyz + 2}}{{\rm{x}}^2} - y + 2\\ M - N = 3xyz - 3{{\rm{x}}^2} + 5{\rm{x}}y - 1 - \left( {5{{\rm{x}}^2} + xyz - 5{\rm{x}}y + 3 - y} \right)\\ = 3xyz - 3{{\rm{x}}^2} + 5{\rm{x}}y - 1 - 5{{\rm{x}}^2} - xyz + 5{\rm{x}}y - 3 + y\\ = 2xyz - 8{{\rm{x}}^2} + 10{\rm{x}}y + y - 4\\ N - M = 5{{\rm{x}}^2} + xyz - 5{\rm{x}}y + 3 - y - \left( {3xyz - 3{{\rm{x}}^2} + 5{\rm{x}}y - 1} \right)\\ = 5{{\rm{x}}^2} + xyz - 5{\rm{x}}y + 3 - y - 3xyz + 3{{\rm{x}}^2} - 5{\rm{x}}y + 1\\ = 8{x^2} - 2{\rm{x}}yz - 10{\rm{x}}y - y + 4 \end{array}\)

-- Mod Toán 7 HỌC247