khó nhìn quá bn ơi

1/4*9+1/9*14+1/14*19+...+1/1999*2004

= 1/5 (1/4 - 1/9 + 1/9 - 1/14 + ... + 1/1999 - 1/2004)

= 1/5 (1/4 - 1/2004)

= 1/5 (501/2004 - 1/2004)

= 1/5 . 125/501

= 25/501

\(\frac{1}{4\times9}+\frac{1}{9\times14}+\frac{1}{14\times19}+...+\frac{1}{1999\times2004}\)

\(=\frac{1}{5}\times\left(\frac{5}{4\times9}+\frac{5}{9\times14}+\frac{5}{14\times19}+...+\frac{5}{1999\times2004}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{1999}-\frac{1}{2004}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{4}-\frac{1}{2004}\right)\)

\(=\frac{1}{5}\times\frac{500}{2004}=\frac{25}{501}\)

Ta có :

A = \(\frac{1}{4.9}+\frac{1}{9.14}+\frac{1}{14.19}+...+\frac{1}{64.69}\)

5A = \(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+...+\frac{5}{64.69}\)

5A = \(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{64}-\frac{1}{69}\)

5A = \(\frac{1}{4}-\frac{1}{69}\)

5A = \(\frac{65}{276}\)

A = \(\frac{65}{276}:5\)

A = \(\frac{13}{276}\)

thiếu

1: =1-1+5-5+7-7+8-8=0

2: =14-23+5+14-5+23+17

=28+17=45

3: =12-12+9-9+14-44-3=-33

4: =22-8-8-12+4

=22-16-8

=-2

Bài \(1\)

\(1)\) \(1-5+7-8+4-1+5-7+8\)

\(=(1-1)+(5-5)+(7-7)+(8-8)\)

\(=0+0+0+0\)

\(=0\)

\(2)\) \(14-23+(5+14)-(5-23)+17\)

\(=14-23+5+14-5+23+17\)

\(=(14+14)+(23-23)+(5-5)+17\)

\(=28+17\)

\(=45\)

\(3)\) \(12-44+9-3+14-19-9-12\)

\(=(12-12)+(9-9)+(14-44)+3\)

\(=-30+3\)

\(=-33\)

\(4)\) \(22-(4-8+12)+(-8-12+4)\)

\(=22-4+8-12-8-12+4\)

\(=22+(4+4)+(8-8)+(-12-12)\)

\(=22-24\)

\(=-2\)

9/14 .<.. 11/14           4/25 .<.. 4/23          5/6 ..<. 7/8         1 ..>. 14/15

8/9 ..=. 24/27             20/19 ..>. 20/27        8/7 .>.. 9/8        1 .<..15/14

\(\dfrac{9}{14}< \dfrac{11}{14};\dfrac{8}{9}=\dfrac{24}{27}\)

\(\dfrac{4}{25}< \dfrac{4}{23};\dfrac{20}{19}>\dfrac{20}{27}\)

\(\dfrac{5}{6}< \dfrac{7}{8};\dfrac{8}{7}>\dfrac{9}{8}\)

\(1>\dfrac{14}{15};1< \dfrac{15}{14}\)

9/14 .<.. 11/14           4/25 .<.. 4/23          5/6 ..<. 7/8         1 ..>. 14/15

8/9 ..=. 24/27             20/19 ..>. 20/27        8/7 .>.. 9/8        1 .<..15/14

-9/28