Thực hiện phép tính: a){x + 3y} + {2x--y}      

a) \[\left( {x + 3y} \right) + \left( {2x--y} \right)\]

\[ = x + 3y + 2x--y\]

\[ = \left( {x + 2x} \right) + \left( {3y--y} \right)\]

\[ = 3x + 2y.\]      

b) \[\left( {x - y} \right)\left( {x - 5y} \right)\]

\[ = {x^2}--5xy--xy + 5{y^2}\]

\[ = {x^2} - 6xy + 5{y^2}\].

c)\[\left( {4{x^3}{y^2}--8{x^2}y + 10xy} \right):\left( {2xy} \right)\]

\[ = 4{x^3}{y^2}:\left( {2xy} \right)--8{x^2}y:\left( {2xy} \right) + 10xy:\left( {2xy} \right)\]

\[ = 2{x^2}y--4x + 5.\]