Question

Bài 30: tính giá trị của các bằng cách tính nhanh nhất

a,21/25 x 11/9 x 5/7

b, 5/23 x 17/26 + 5/23 x 9/26

Answer 1

a) Ta có: \(\dfrac{21}{25}\cdot\dfrac{11}{9}\cdot\dfrac{5}{7}\)

\(=\dfrac{21\cdot11\cdot5}{25\cdot9\cdot7}\)

\(=\dfrac{3\cdot7\cdot11\cdot5}{5\cdot5\cdot3\cdot3\cdot7}\)

\(=\dfrac{11}{5\cdot3}=\dfrac{11}{15}\)

Answer 2

a) 21/25. 11/9. 5/7

=(21/25.5/7).11/9

= 3/5. 11/9

=11/15

b) 5/23 . 17/26 . 5/23 . 9/26

=5/23.( 17/26+9/26)

=5/23. 1

=5/23

Answer 3

b) Ta có: \(\dfrac{5}{23}\cdot\dfrac{17}{26}+\dfrac{5}{23}\cdot\dfrac{9}{26}\)

\(=\dfrac{5}{23}\left(\dfrac{17}{26}+\dfrac{9}{26}\right)\)

\(=\dfrac{5}{23}\cdot1=\dfrac{5}{23}\)