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- Hướng dẫn giải

a) Ta có $\frac{2{\mathrm{u}}^2-20\mathrm{u}+50}{5\mathrm{u}+5}.\frac{2{\mathrm{u}}^2-2}{4{(\mathrm{u}-5)}^3}$$=\frac{2{(\mathrm{u}-5)}^2}{5(\mathrm{u}+1)}.\frac{2(\mathrm{u}-1)(\mathrm{u}+1)}{4{(\mathrm{u}-5)}^3}=\frac{\mathrm{u}-1}{5(\mathrm{u}-5)}$ 

b) Ta có $\frac{\mathrm{v}+3}{{\mathrm{v}}^2-4}.\frac{8-12\mathrm{v}+6{\mathrm{v}}^2-{\mathrm{v}}^3}{7\mathrm{v}+21}=\frac{\mathrm{v}+3}{(\mathrm{v}-2)(\mathrm{v}+2)}.\frac{{(2-\mathrm{v})}^3}{7(\mathrm{v}+3)}$$=\frac{1}{(\mathrm{v}-2)(\mathrm{v}+2)}.\frac{-{(\mathrm{v}-2)}^3}{7}=-\frac{{(\mathrm{v}-2)}^2}{7(\mathrm{v}+2)}$