Một tấm kính hình vuông có chu vi là 5 m. Hỏi diện tích tấm kính đó bằng bao nhiêu mét vuông?

a) \(\frac{2}{5} + \frac{3}{{10}}\) = \(\frac{7}{{10}}\)

Giải thích:

\(\frac{2}{5}\,\, + \,\,\frac{3}{{10}}\,\, = \,\,\frac{4}{{10}}\,\, + \,\,\frac{3}{{10}}\,\, = \,\,\frac{7}{{10}}\)

Vì \(\frac{7}{{10}}\,\, = \,\,\frac{7}{{10}}\) nên \(\frac{2}{5} + \frac{3}{{10}}\) = \(\frac{7}{{10}}\)

b) \(\frac{4}{9}\) < \(\frac{2}{9} + \frac{1}{3}\)

Giải thích:

\(\frac{2}{9}\,\, + \,\,\frac{1}{3}\,\, = \,\,\frac{2}{9}\,\, + \,\,\frac{3}{9}\,\, = \,\,\frac{5}{9}\)

Vì \(\frac{4}{9}\,\, < \,\,\frac{5}{9}\) nên \(\frac{4}{9}\) < \(\frac{2}{9} + \frac{1}{3}\)

c) \(\frac{5}{8} + \frac{1}{4}\) < \(\frac{3}{4}\)

Giải thích:

\(\frac{5}{8}\,\, + \,\,\frac{1}{4}\,\, = \,\,\frac{5}{8}\,\, + \,\,\frac{2}{8}\,\, = \,\,\frac{7}{8}\)

\(\frac{3}{4}\,\, = \,\,\frac{6}{8}\)

Vì \(\frac{7}{8}\,\, < \,\,\frac{6}{8}\) nên \(\frac{5}{8} + \frac{1}{4}\) < \(\frac{3}{4}\)

d) \(\frac{7}{{12}}\) = \(\frac{1}{3} + \frac{1}{4}\)

Giải thích:

\(\frac{1}{3}\,\, + \,\,\frac{1}{4}\,\, = \,\,\frac{4}{{12}}\,\, + \,\,\frac{3}{{12}}\,\, = \,\,\frac{7}{{12}}\)

Vì \(\frac{7}{{12}}\,\, = \,\,\frac{7}{{12}}\)

Nên \(\frac{7}{{12}}\) = \(\frac{1}{3} + \frac{1}{4}\)

a) \(\frac{{18}}{{45}} + 4 \times \frac{1}{6}\)

= \(\frac{2}{5} + 4 \times \frac{1}{6}\)

= \(\frac{2}{5} + \frac{4}{6}\) = \(\frac{2}{5} + \frac{2}{3}\) = \(\frac{{16}}{{15}}\)

b) \(\frac{{12}}{8} \times (\frac{5}{{25}}:\frac{{20}}{{15}})\)

= \(\frac{3}{2} \times (\frac{1}{5}:\frac{4}{3})\)

= \(\frac{3}{2} \times (\frac{1}{5} \times \frac{3}{4})\) = \(\frac{3}{2} \times \frac{3}{{20}}\) = \(\frac{9}{{40}}\)