Bài 1: Ta có:
\(\eqalign{ A - B + C &= (2{{\rm{a}}^2} - 3{\rm{a}}b + 4{b^2}) - (3{{\rm{a}}^2} + 4{\rm{a}}b - {b^2}) + ({a^2} + 2{\rm{a}}b + 3{b^2}) \cr & {\rm{ }} = 2{{\rm{a}}^2} - 3{\rm{a}}b + 4{b^2} - 3{{\rm{a}}^2} - 4{\rm{a}}b + {b^2} + {a^2} + 2{\rm{a}}b + 3{b^2} \cr & {\rm{ }} = - 5{\rm{a}}b + 8{b^2}. \cr} \)
Bài 2: Ta có:
\(\eqalign{ M &= 3{{\rm{x}}^2} + 5{\rm{x}}y + 7{{\rm{x}}^2}y - (5{\rm{x}}y + 3{{\rm{x}}^2} - 7{{\rm{x}}^2}y + 3{x^2}) \cr & {\rm{ }} = {{\rm{x}}^2} + 5{\rm{x}}y + 7{{\rm{x}}^2}y - 5{\rm{x}}y - 3{{\rm{x}}^2} + 7{{\rm{x}}^2}y - 3{x^2} \cr & {\rm{ }} = - 3{{\rm{x}}^2} + 14{{\rm{x}}^2}y. \cr} \)
Bài 3: Ta có:
\(P = 2{{\rm{x}}^2}{y^2} + 2{x^3}y - xy + x{y^3} + 3{\rm{x}}y - 3{{\rm{x}}^3}y + x{y^3} - {x^2}{y^2} \)
\(\;\;\;\;= {x^2}{y^2} - {x^3}y + 2{\rm{x}}y + 2x{y^3}.\)
