\(\frac{3}{4}\) của 1 thế kỉ là:  

Đáp án đúng là: C

\(\frac{4}{5}\) m² \(\frac{3}{4}\) cm² = …………………. mm²

Ta có:

\[\frac{4}{5}{m^2}\frac{3}{4}c{m^2} = \frac{4}{5} \times 1\,\,000\,\,000\,m{m^2} + \frac{3}{4} \times 100\,m{m^2}\]

                         \[ = 800\,\,000\,m{m^2} + 75\,m{m^2}\]

                         = 800 075 mm²

Vậy số cần điền là: 800 075

\(\frac{5}{2}\,\, \times \,\,\frac{3}{5}\,\, + \,\,\frac{5}{2}\,\, \times \,\,\frac{1}{5}\,\, + \,\,\frac{5}{2}\)

\( = \frac{5}{2}\,\, \times \,\,\left( {\frac{3}{5}\,\, + \,\,\frac{1}{5}\,\, + \,1} \right)\)

\( = \frac{5}{2}\,\, \times \,\,\left( {\frac{3}{5}\,\, + \,\,\frac{1}{5}\,\, + \,\frac{5}{5}} \right)\)

\( = \frac{5}{2}\,\, \times \,\frac{9}{5}\)

\( = \frac{{\not 5}}{2}\,\, \times \,\frac{9}{{\not 5}}\)

\( = \frac{9}{2}\,\)