Giải câu 1 trang 51 toán VNEN 8 tập 1

a) $\frac{-11x^{3}}{8y^{4}}$.$\frac{12y^{2}}{55x}$ = $\frac{-11x^{3}.12y^{2}}{8y^{4}.55x}$ = $\frac{-11.4.3x^{3}y^{2}}{4.2.11.5xy^{4}}$ = $\frac{-3x^{2}}{10y^{2}}$; 

b) $\frac{48x^{6}}{125y^{6}}$.(-$\frac{50y^{4}}{9x^{3}}$) = $\frac{48x^{6}.(-50y^{4})}{125y^{6}.9x^{3}}$ = $\frac{48.(-50).x^{6}y^{4}}{125.9.x^{3}y^{6}}$ = $\frac{-32x^{3}}{15y^{2}}$; 

c) -3x$^{2}$y$^{2}$.(-$\frac{7}{9xy^{2}}$) = $\frac{-3x^{2}y^{2}}{1}$.($\frac{-7}{9xy^{2}}$) = $\frac{(-3x^{2}y^{2}).(-7)}{9xy^{2}}$ = $\frac{7x}{3}$; 

d) $\frac{3y}{28x^{2}}$.$\frac{2x}{7y^{4}}$.49x$^{4}$y$^{3}$ = $\frac{3y.2x.49x^{4}y^{3}}{28x^{2}.7y^{4}}$ = $\frac{3.2.49x^{5}y^{4}}{28.7.x^{2}y^{4}}$ = $\frac{3x^{3}}{2}$.