Ta có : \(\frac{1}{2}+\frac{5}{14}+\frac{2}{63}+\frac{3}{108}+\frac{1}{156}\)

\(=\frac{1}{1.2}+\frac{5}{2.7}+\frac{2}{7.9}+\frac{3}{9.12}+\frac{1}{12.13}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}\)

\(=1-\frac{1}{13}=\frac{12}{13}\)

a: 12/21=4/7=8/14

4/56=1/14

b: 12/15=4/5=24/30

6/36=1/6=5/30

15/50=3/10=9/30

c: 63/77=9/11=81/99

60/108=5/9=55/99

-18/27=-2/3=-66/99

d: -26/-156=1/6=7/42

49/-14=-7/2=-147/42

32/112=2/7=12/42

`a)2\sqrt{28}+\sqrt{63}-\sqrt{112}`

`=4\sqrt{7}+3\sqrt{7}-4\sqrt{7}`

`=3\sqrt{7}`.

`b)3\sqrt{48}-5\sqrt{108}+6\sqrt{1/3}`

`=12\sqrt{3}-30\sqrt{3}+2\sqrt{[3^2]/3}`

`=-18\sqrt{3}+2\sqrt{3}`

`=-16\sqrt{3}`.

27.75 +25.27 -150 

= 27. ( 75+25 ) -150      

= 27.100 - 150 

= 2700 - 150 = 2550 

2, 34.35 -25 .12 + 105 

= 1190 - 300 + 105 

= 890 + 105 = 995

  51.8 + 25.15 - 63 

=408 + 375 - 63 

= 783 - 63 

= 720

4,918 : 9 + 20 . 46 - 132 

=102 + 20 . 46 - 132 

= 102 + 920 - 132 

= 1022 - 132 

= 890 

52 .47 - 35 .16 + 230 

= 2444 - 560 + 230 

= 1884 + 230 

= 2114 

156 + 38 .64 - 36 . 21 

= 156 + 2432 - 756 

= 2588 - 756 = 1832 

A = \(\dfrac{2}{3}\) + \(\dfrac{3}{18}\) + \(\dfrac{1}{42}\) + \(\dfrac{2}{63}\) + \(\dfrac{3}{108}\)

A = \(\dfrac{2}{1\times3}\) + \(\dfrac{3}{3\times6}\) + \(\dfrac{1}{6\times7}\)+ \(\dfrac{2}{7\times9}\) + \(\dfrac{3}{9\times12}\)

A = \(\dfrac{1}{1}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{6}\) + \(\dfrac{1}{6}\) - \(\dfrac{1}{7}\) + \(\dfrac{1}{7}\) - \(\dfrac{1}{9}\) + \(\dfrac{1}{9}\) - \(\dfrac{1}{12}\)

A = 1 - \(\dfrac{1}{12}\)

A = \(\dfrac{11}{12}\) 

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