15: A= 1/3-3/4+3/5+1/2007-1/36+1/15-2/9

Sửa đề: 

A=-3/4-2/9-1/36+1/3+3/5+1/15+1/2007

=-27/36-8/36-1/36+5/15+9/15+1/15+1/2007

=-1+1+1/2007=1/2007

16:

\(A=\dfrac{1}{3}+\dfrac{3}{5}+\dfrac{1}{15}-\dfrac{3}{4}-\dfrac{2}{9}-\dfrac{1}{36}+\dfrac{1}{64}\)

\(=\dfrac{5+9+1}{15}+\dfrac{-27-8-1}{36}+\dfrac{1}{64}\)

=1/64

17:

=1/2-1/2+2/3-2/3+3/4-3/4+4/5-4/5+5/6-5/6-6/7

=-6/7

\(B=\left(\dfrac{-5}{3}+\dfrac{2}{5}\right):\dfrac{4}{5}+\left(\dfrac{-1}{3}+\dfrac{8}{5}\right):\dfrac{4}{5}\)

\(B=\dfrac{-5}{3}+\dfrac{2}{5}:\dfrac{4}{5}+\dfrac{-1}{3}+\dfrac{8}{5}:\dfrac{4}{5}\)

\(B=\left(\dfrac{-5}{3}+\dfrac{-1}{3}\right)+\left(\dfrac{2}{5}+\dfrac{8}{5}\right):\dfrac{4}{5}\)

\(B=\left[\left(-2\right)+2\right]:\dfrac{4}{5}\)

\(B=0:\dfrac{4}{5}=0\)

\(=>B=\dfrac{5}{4}\cdot\left(-\dfrac{5}{3}+\dfrac{2}{5}-\dfrac{1}{3}+\dfrac{8}{5}\right)=\dfrac{5}{4}\cdot\left(-2+2\right)=\dfrac{5}{4}\cdot0=0\)

\(=>C=\left(-\dfrac{2}{3}+\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{2}\right)+\left(\dfrac{1}{69}-\dfrac{1}{36}-\dfrac{2}{9}\right)\)

\(C=\left(-\dfrac{2}{6}+\dfrac{1}{6}+\dfrac{3}{6}\right)-\dfrac{65}{276}=\dfrac{1}{3}-\dfrac{65}{276}=\dfrac{157}{276}\)

`5/2 - 2/3 + 5/4`

`= 30/12 - 8/12 + 15/12`

`= (30-8+15)/12`

`= 37/12`

`----`

`20/12 + 36/18 -21/28`

`= 5/3 + 2-3/4`

`= 20/12 + 24/12 -9/12`

`= 35/12`

`5/2-2/3+5/4`

`=30/12-8/12+15/12`

`=37/12`

`20/12+36/18-21/28`

`=5/3+2-3/4`

`=20/12+24/12-9/12`

`=35/12`

Bài 2. > , < ,  = ?

5/7 < 4/3             2/5 < 6/10            1/4 = 3/12                27/36 >

2/9           7/6 > 7/9

7/2 = 2/7        15/23 < 1                  27/9 > 2          14/15 < 1          51/17 < 4

sai rôi

`2x -2/3 +1/2x =-1`

`=> 2x+1/2x =-1+2/3`

`=> (2+1/2) x =-3/3 + 2/3`

`= 5/2 x = -1/3`

`=> x=-1/3 : 5/2`

`=>x= -2/15`

`31/36 - (1/3-x)^2 =5/6`

`=>  (1/3-x)^2 = 31/36 - 5/6`

`=>  (1/3-x)^2 =1/36`

`=>  (1/3-x)^2 = (+-1/6)^2`

\(\Rightarrow\left[{}\begin{matrix}\dfrac{1}{3}-x=\dfrac{1}{6}\\\dfrac{1}{3}-x=-\dfrac{1}{6}\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=\dfrac{1}{6}\\x=\dfrac{1}{2}\end{matrix}\right.\)

a,     (\(\dfrac{9}{10}\) - \(\dfrac{15}{16}\)) \(\times\) ( \(\dfrac{5}{12}\) - \(\dfrac{11}{15}\) - \(\dfrac{7}{20}\))

=  (\(\dfrac{72}{80}\) - \(\dfrac{75}{80}\))  \(\times\) (\(\)\(\dfrac{25}{60}\) - \(\dfrac{44}{60}\)  - \(\dfrac{21}{60}\))

= - \(\dfrac{3}{80}\)  \(\times\) (- \(\dfrac{2}{3}\))

= \(\dfrac{1}{40}\) 

b, (-1)³ + (- \(\dfrac{2}{3}\))² : 2\(\dfrac{2}{3}\) + \(\dfrac{5}{6}\)

=  -1³ +   \(\dfrac{4}{9}\) : \(\dfrac{8}{3}\) + \(\dfrac{5}{6}\)

= -1 + \(\dfrac{4}{9}\) \(\times\) \(\dfrac{3}{8}\) + \(\dfrac{5}{6}\)

= -1 + \(\dfrac{1}{6}\) + \(\dfrac{5}{6}\)
= -1 + 1

= 0

c, 15 \(\times\) (- \(\dfrac{1}{5}\))² + \(\dfrac{1}{5}\) - 2 \(\times\) (- \(\dfrac{1}{2}\))³ - \(\dfrac{1}{2}\)

= 15 \(\times\) \(\dfrac{1}{25}\) + \(\dfrac{1}{5}\) - 2 \(\times\) (- \(\dfrac{1}{8}\)) - \(\dfrac{1}{2}\)

=  \(\dfrac{3}{5}\)  + \(\dfrac{1}{5}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{2}\)

= \(\dfrac{4}{5}\) - \(\dfrac{1}{4}\)

= \(\dfrac{11}{20}\)