a) Thu gọn đơn thức: \(\frac{{ - 3}}{4}{x^2}y.

a) \(\frac{{ - 3}}{4}{x^2}y.{\left( {\frac{{ - 2}}{3}z{y^2}{x^3}} \right)^2} = \frac{{ - 3}}{4}{x^2}y.\frac{4}{9}{z^2}{y^4}{x^6} = \frac{{ - 1}}{3}{x^8}{y^5}{z^2}\)

b) A = – 5x² – 6xy + 3y² – (4x² – 6xy + y²)

        = – 5x² – 6xy + 3y² – 4x² + 6xy – y²

        = – 9x² +  2y²

Tại \(x =\frac{5}{3}\) và \(y = \frac{{ - 1}}{2}\) ta có: A = \( - 9.{\left( {\frac{5}{3}} \right)^2} + 2.{\left( { - \frac{1}{2}} \right)^2} =  - 9.\frac{{25}}{9} + 2.\frac{1}{4} =  - 25 + \frac{1}{2} = \frac{{ - 49}}{2}\)