\(\displaystyle {\rm{a}})2{{\rm{x}}^2}yz + 4{\rm{x}}{y^2}z\)\(\displaystyle - 5{{\rm{x}}^2}yz + x{y^2}z - xyz \)
\(\displaystyle = (2 - 5){x^2}yz + (4 + 1)x{y^2}z - xyz \)\(\displaystyle = - 3{{\rm{x}}^2}yz + 5{\rm{x}}{y^2}z - xyz \)
\(\displaystyle b){x^3} - 5{\rm{x}}y + 3{{\rm{x}}^3} + xy - {x^2} \)\(\displaystyle + {1 \over 2}xy - {x^2} \)
\(\displaystyle = (1 + 3){x^3} - \left( {5 - 1 - {1 \over 2}} \right)xy\)\(\displaystyle - (1 + 1){x^2} \)
\(\displaystyle = 4{{\rm{x}}^3} - {7 \over 2}xy - 2{{\rm{x}}^2} \)
