Bài giải :

$A=\frac{8}{9}.\frac{15}{16}.\frac{24}{25}.\frac{35}{36}.\frac{48}{49}.\frac{63}{64}=\frac{{{2}^{3}}}{{{3}^{2}}}.\frac{3.5}{{{2}^{4}}}.\frac{{{2}^{3}}.3}{{{5}^{2}}}.\frac{5.7}{{{2}^{2}}{{.3}^{2}}}.\frac{{{2}^{4}}.3}{{{7}^{2}}}.\frac{{{3}^{2}}.7}{{{2}^{6}}}$

$A=\frac{{{2}^{10}}{{.3}^{5}}{{.5}^{2}}{{.7}^{2}}}{{{2}^{12}}{{.3}^{4}}{{.5}^{2}}{{.7}^{2}}}=\frac{3}{4}$