a) 145 + 86 + 14 + 55 = ( 145 + 55 ) + (86 + 14 )

= 200 + 100

= 300

b) 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = ( 1+9 +2 + 8) + ( 3 + 7 + 4 + 6 ) + 5

= 20 + 20 + 5

= 45

\(\frac{3}{8}.\frac{5}{7}+\frac{6}{8}.\frac{1}{14}+\frac{9}{8}.\frac{1}{21}\)

\(=\frac{3}{8}.\left(\frac{5}{7}+2.\frac{1}{14}+3.\frac{1}{21}\right)\)

\(=\frac{3}{8}.\left(\frac{5}{7}+\frac{1}{7}+\frac{1}{7}\right)\)

\(=\frac{3}{8}.1\)

\(=\frac{3}{8}\)

\(\dfrac{21}{16}\times\dfrac{8}{5}\times\dfrac{15}{7}=\dfrac{21\times8\times15}{16\times5\times7}=\dfrac{7\times3\times8\times5\times3}{8\times2\times5\times7}=\dfrac{3\times3}{2}=\dfrac{9}{2}\)

\(\dfrac{18}{9}\times\dfrac{9}{36}\times\dfrac{27}{3}=2\times\dfrac{1}{4}\times9=\dfrac{1}{2}\times9=\dfrac{9}{2}\)

\(\frac{5x8x24x27}{8x15x9x7}=\frac{5x8x24x9x3}{8x5x3x9x7}=\frac{24}{7}\)

Chúc bạn học tốt.

😁😁😁

Ta tính thuận tiện bằng cách phân tích các thừa số để bỏ thừa số chung:

\(\frac{5\text{x}8\text{x}24\text{x}27}{8\text{x}15\text{x}9\text{x}7}=\frac{5\text{x}8\text{x}3\text{x}8\text{x}3\text{x}3\text{x}3}{8\text{x}3\text{x}5\text{x}3\text{x}3\text{x}7}\)

\(=\frac{8\text{x}3}{7}\)

\(=\frac{24}{7}\)

\(\left(\dfrac{2}{7}+\dfrac{5}{7}\right)+\left(\dfrac{3}{5}+\dfrac{2}{5}\right)+\left(\dfrac{3}{8}+\dfrac{4}{8}+\dfrac{1}{8}\right)+\left(\dfrac{1}{9}+\dfrac{8}{9}\right)=1+1+1+1=4\)

(2/7 + 5/7) + (3/5 + 2/5) + (3/8 + 4/8 + 1/8) + (1/9 + 8/9 )

= 1  +       1                   +         1             +        1         =  4

\(\dfrac{14}{9}\) x \(\dfrac{11}{8}-\dfrac{3}{8}\) x \(\dfrac{14}{9}\)

\(=\dfrac{14}{9}\) x \(\left(\dfrac{11}{8}-\dfrac{3}{8}\right)\)

\(=\dfrac{14}{9}\) x \(\dfrac{8}{8}=\dfrac{14}{9}\) x \(1\)

\(=\dfrac{14}{9}\)

oa cảm ơn bạn nhé :3

14/9 × 11/8 - 3/8 × 14/9 

= 14/9 × ( 11/8 - 3/8 ) 

= 14/9 × 8/8 

= 14/9 - 1 

= 5/9

\(=\left(\dfrac{3}{7}+\dfrac{8}{14}\right)+\left(\dfrac{3}{10}+\dfrac{7}{10}\right)=1+1=2\)

Mtbrvrvsdqdqd

`a: =(7/3-4/3)+11/5=16/5`

`b: =(18/13-5/13)+55/46=1+55/46=101/46`

`c: =27/25-4/9-2/25+5/9=(27/25-2/25)+(5/9-4/9)=1+1/9=10/9`