Question

Tính bằng cách thuận tiện nhất

a)3/5x17/21+17/21x2/5

b)15/8:5/20-7/8:1/4

Answer 1

a) \(\dfrac{3}{5}\times\dfrac{17}{21}+\dfrac{17}{21}\times\dfrac{2}{5}\)

\(=\dfrac{17}{21}\times\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\)

\(=\dfrac{17}{21}\times1\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}\times\dfrac{20}{5}-\dfrac{7}{8}\times4\)

\(=\dfrac{15}{8}\times4-\dfrac{7}{8}\times4\)

\(=4\times\left(\dfrac{15}{8}-\dfrac{7}{8}\right)\)

\(=4\times1\)

\(=4\)

Answer 2

a, =(3/5+2/5)x17/21

=1x17/21

=17/21

Answer 3

a) \(\dfrac{3}{5}\) x \(\dfrac{17}{21}+\dfrac{17}{21}\) x \(\dfrac{2}{5}\)

\(=\left(\dfrac{3}{5}+\dfrac{2}{5}\right)\) x \(\dfrac{17}{21}\)

\(=\dfrac{5}{5}\) x \(\dfrac{17}{21}\)

\(=1\) x \(\dfrac{17}{21}\)

\(=\dfrac{17}{21}\)

b) \(\dfrac{15}{8}:\dfrac{5}{20}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\dfrac{15}{8}:\dfrac{1}{4}-\dfrac{7}{8}:\dfrac{1}{4}\)

\(=\left(\dfrac{15}{8}-\dfrac{7}{8}\right):\dfrac{1}{4}\)

\(=\dfrac{8}{8}:\dfrac{1}{4}\)

\(=1:\dfrac{1}{4}\)

\(=\dfrac{1}{1}:\dfrac{1}{4}\)

\(=\dfrac{1}{1}\) x \(\dfrac{4}{1}\)

\(=\dfrac{4}{1}\)

\(=4\)