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18 tháng 8 2023

A = \(\dfrac{1}{1+2}\) + \(\dfrac{1}{1+2+3}\) +...+ \(\dfrac{1}{1+2+3+...+9}\)

A = \(\dfrac{1}{\left(2+1\right)\times2:2}\)\(\dfrac{1}{\left(1+3\right)\times3:2}\)+...+ \(\dfrac{1}{\left(1+9\right)\times9:2}\)

A = \(\dfrac{2}{2\times\left(1+2\right)}\)\(\dfrac{2}{3\times\left(1+4\right)}\)+...+ \(\dfrac{2}{9\times\left(1+9\right)}\)

A = \(\dfrac{2}{2\times3}\) + \(\dfrac{2}{3\times4}\)+....+ \(\dfrac{2}{9\times10}\)

A = 2 \(\times\)\(\dfrac{1}{2\times3}\) + \(\dfrac{1}{3\times4}\)+...+ \(\dfrac{1}{9\times10}\))

A = 2 \(\times\) ( \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\)+...+ \(\dfrac{1}{9}\) - \(\dfrac{1}{10}\))

A = 2 \(\times\)\(\dfrac{1}{2}\) - \(\dfrac{1}{10}\))

A = 2 \(\times\) \(\dfrac{2}{5}\)

A = \(\dfrac{4}{5}\)

14 tháng 11 2018

1)A=987

16 tháng 11 2023

a) 1/4(x-3)+2=1/5

1/4.(x-3) = 1/5-2

1/4.(x-3) = -9/5

x-3 = (-9/5):1/4

x-3 = -36/5

x = -36/5+3

x= -21/5

2:

a: =4+3/8+5+2/3

=9+3/8+2/3

=216/24+9/24+16/24

=216/24+25/24

=241/24

b; =2+3/8+1+1/4+3+6/7

=6+3/8+1/4+6/7

=6+5/8+6/7

=419/56

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

=6+3/8-1/4+1/3

=6+1/8+1/3

=6+11/24

=155/24

d: \(=3+\dfrac{5}{6}+6\cdot\dfrac{13}{6}\)

=3+13+5/6

=16+5/6

=101/6

e: =3+1/2+4+5/7-5-5/14

=3+4-5+1/2+5/7-5/14

=2+7/14+10/14-5/14

=2+12/14

=2+6/7=20/7

f: =9/2+1/2:11/2

=9/2+1/11

=99/22+2/22=101/22

2:

a: =4+3/8+5+2/3

=9+3/8+2/3

=216/24+9/24+16/24

=216/24+25/24

=241/24

b; =2+3/8+1+1/4+3+6/7

=6+3/8+1/4+6/7

=6+5/8+6/7

=419/56

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

=6+3/8-1/4+1/3

=6+1/8+1/3

=6+11/24

=155/24

d: \(=3+\dfrac{5}{6}+6\cdot\dfrac{13}{6}\)

=3+13+5/6

=16+5/6

=101/6

e: =3+1/2+4+5/7-5-5/14

=3+4-5+1/2+5/7-5/14

=2+7/14+10/14-5/14

=2+12/14

=2+6/7=20/7

f: =9/2+1/2:11/2

=9/2+1/11

=99/22+2/22=101/22

26 tháng 3 2020

a. \(\left[\left(-2\right)^5.2014-4^2.2015\right]-\left(-2015^0+3^2-2^3\right)\)

\(=-64448-32240+1-9+8=-96688\)

27 tháng 3 2020

Tớ lm lại nhé:

SBC = 9-1/2-1/3-1/4-...-1/10

=1+1+...+1(9 số 1) -1/2-1/3-1/4-1/5-...-1/10.

=(1-1/2)+(1-1/3)+...+(1-1/10)

=1/2+2/3+...+9/10= SC

=> phép chia có thương là 1(vì SBC=SC)

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

1:

a: =23/27-11/17+4/27+28/17

=23/27+4/27+28/17-11/17

=1+1=2

b: \(=\dfrac{2}{3}\cdot\left(\dfrac{7}{9}+\dfrac{2}{9}\right)-\dfrac{2}{9}\)

=2/3-2/9

=6/9-2/9

=4/9

c: \(=\dfrac{11}{5}\cdot\dfrac{7}{3}-\dfrac{1}{3}\cdot\dfrac{11}{5}\)

=11/5(7/3-1/3)

=11/5*2

=22/5

d: \(=\dfrac{3}{2}\cdot\dfrac{4}{3}\cdot...\cdot\dfrac{2024}{2023}=\dfrac{2024}{2}=1012\)

e: \(=\dfrac{1}{2}\cdot\dfrac{2}{3}\cdot...\cdot\dfrac{2022}{2023}=\dfrac{1}{2023}\)

 

18 tháng 8 2023

thiếu rồi bạn ơi

20 tháng 8 2023

 

A= 1+ \(\dfrac{1}{2}\) (1+2) + \(\dfrac{1}{3}\) (1+2+3)+.....+

\(\dfrac{1}{9}\) (1+2+3+....+9)

=1+ \(\dfrac{1}{2}\) ×2×3 + \(\dfrac{1}{3}\) × \(\dfrac{3×4}{2}\)+.....+

\(\dfrac{1}{9}\) × \(\dfrac{9×10}{2}\)

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

\(\dfrac{10}{2}\)

=\(\dfrac{2}{2}\) + \(\dfrac{3}{2}\) + \(\dfrac{4}{2}\) +......+

\(\dfrac{10}{2}\)

=\(\dfrac{2+3+4+...10}{2}\)

=\(\dfrac{12×10}{2}{2}\)

= \(\dfrac{12×10}{4}\)

= 12×5 = tự nhân ra 

Vậy A bằng ở trên .

Nhớ tích mình nhé:)