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19 tháng 6 2019

\(B=\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{73.76}\)

\(\Leftrightarrow B=\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{73}-\frac{1}{76}\)

\(\Leftrightarrow B=\frac{1}{4}+\left(\frac{1}{7}-\frac{1}{7}\right)+\left(\frac{1}{10}-\frac{1}{10}\right)+...+\left(\frac{1}{73}-\frac{1}{73}\right)-\frac{1}{76}\)

\(\Leftrightarrow B=\frac{1}{4}-\frac{1}{76}=\frac{9}{38}\)

~ Hok tốt ~

19 tháng 6 2019

\(\left[200-18:\left(372:3.x-1\right)\right]-28=166\)

\(\Leftrightarrow200-18:\left(124.x-1\right)=166+28\)

\(\Leftrightarrow200-18:\left(124.x-1\right)=194\)

\(\Leftrightarrow18:\left(372:3.x-1\right)=200-194\)

\(\Leftrightarrow18:\left(124.x-1\right)=6\)

\(\Leftrightarrow124.x-1=18:6\)

\(\Leftrightarrow124.x-1=3\)

\(\Leftrightarrow124.x=3+1\)

\(\Leftrightarrow124.x=4\)

\(\Leftrightarrow x=4:124\)

\(\Leftrightarrow x=\frac{1}{31}\)

~ Hok tốt ~

21 tháng 7 2016

1.      \(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{40.43}\)

\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{40}-\frac{1}{43}\)

\(=1-\frac{1}{43}\)

\(=\frac{42}{43}\)

2.     Đặt \(A=\frac{2}{2}+\frac{2}{6}+\frac{2}{12}+...+\frac{2}{90}\)

\(=2.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\right)\)

\(=2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\right)\)

\(=2.\left(1-\frac{1}{10}\right)\)

\(=2.\frac{9}{10}\)

\(=\frac{9}{5}\)

Ủng hộ mk nha !!! ^_^

21 tháng 7 2016

1) 3/1×4 + 3/4×7 + 3/7×10 + ... + 3/40×43

= 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + ... + 1/40 - 1/43

= 1 - 1/43

= 42/43

2) 2/2 + 2/6 + 2/12 + ... + 2/90

= 2 × (1/2 + 1/6 + 1/12 + ... + 1/90)

= 2 × (1/1×2 + 1/2×3 + 1/3×4 + ... + 1/9×10)

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

= 2 × (1 - 1/10)

= 2 × 9/10

= 9/5

6 tháng 5 2018

S=3/2^0+3/2^1+....+3/2^2018

S=3/2.(2/2^0+2/2^1+....+2^2018)

đặt B=2/2^0+2/2^1+....+2^2018

2B=2.(2/2^0+2/2^1+....+2^2018)

2B=1+2/2^0+...+2/2^2017

2B-B=(1+2/2^0+...+2/2^2017)-(2/2^0+2/2^1+....+2^2018)

B=1-2^2018

S=3/2.1-2^2018=3/2^2018

6 tháng 5 2018

B=2^2018-1 nha mink làm lộn

31 tháng 7 2018

\(\frac{1}{3}\) + \(\frac{5}{6}\)\(\left(x-2\frac{1}{5}\right)\)\(\frac{3}{4}\)

<=> \(\frac{5}{6}\):\(\left(x-2\frac{1}{5}\right)\)\(\frac{3}{4}\)\(\frac{1}{3}\)

<=> \(\frac{5}{6}\) : \(\left(x-2\frac{1}{5}\right)\) = \(\frac{5}{12}\)

<=> \(\left(x-2\frac{1}{5}\right)\) =    \(\frac{5}{6}\) : \(\frac{5}{12}\)

,<=> \(\left(x-2\frac{1}{5}\right)\)=   2 

<=. x = 2 + \(\frac{11}{5}\)

<=> x = \(\frac{21}{5}\)

7 tháng 2 2018

ĐKXĐ : \(\hept{\begin{cases}x^2+x-6\ne0\\x^2+4x+3\ne0\\2x-1\ne0\end{cases}\Leftrightarrow\hept{\begin{cases}\left(x+3\right)\left(x-2\right)\ne0\\\left(x+1\right)\left(x+3\right)\ne0\\x\ne\frac{1}{2}\end{cases}\Rightarrow\hept{\begin{cases}x\ne2;-3\\x\ne-1;-3\\x\ne\frac{1}{2}\end{cases}}}}\)

TXĐ : \(x\ne\left\{-3;-1;\frac{1}{2};2\right\}\)

\(pt\Leftrightarrow\frac{5}{\left(x+3\right)\left(x-2\right)}-\frac{2}{\left(x+1\right)\left(x+3\right)}=\frac{-3}{2x-1}\)

\(\Leftrightarrow\frac{5\left(x+1\right)-2\left(x-2\right)}{\left(x-2\right)\left(x+1\right)\left(x+3\right)}=\frac{-3}{2x-1}\)

\(\Leftrightarrow\frac{3x+9}{\left(x-2\right)\left(x+1\right)\left(x+3\right)}=\frac{-3}{2x-1}\)

\(\Leftrightarrow\frac{3}{\left(x-2\right)\left(x+1\right)}=\frac{-3}{2x-1}\)

\(\Leftrightarrow\frac{1}{x^2-x-2}=\frac{1}{1-2x}\)

\(\Leftrightarrow x^2-x-2-1+2x=0\)

\(\Leftrightarrow x^2+x-3=0\)

\(\Leftrightarrow\left(x^2+2.\frac{1}{2}.x+\frac{1}{4}\right)-\frac{13}{4}=0\)

\(\Leftrightarrow\left(x+\frac{1}{2}\right)^2-\left(\frac{\sqrt{13}}{2}\right)^2=0\)

\(\Leftrightarrow\left(x+\frac{1-\sqrt{13}}{2}\right)\left(x+\frac{1+\sqrt{13}}{2}\right)=0\)

\(\Rightarrow\orbr{\begin{cases}x=\frac{\sqrt{13}-1}{2}\\x=\frac{-\sqrt{13}-1}{2}\end{cases}}\)

7 tháng 2 2018

\(\frac{5}{x^2+x-6}-\frac{2}{x^2+4+3}=-\frac{3}{2x-1}\)

<=> \(\frac{5}{\left(x-2\right)\left(x+3\right)}-\frac{2}{\left(x+1\right)\left(x+3\right)}=-\frac{3}{2x-1}\)

<=> \(\frac{5\left(x+1\right)-2\left(x-2\right)}{\left(x-2\right)\left(x+3\right)}=-\frac{3}{2x-1}\)

<=> \(\frac{5x+5-2x+4}{\left(x-2\right)\left(x+3\right)}=-\frac{3}{2x-1}\)

<=> \(\frac{3x+9}{\left(x-2\right)\left(x+3\right)}=-\frac{3}{2x-1}\)

<=> \(\frac{3\left(x+3\right)}{\left(x-2\right)\left(x+3\right)}=-\frac{3}{2x-1}\)

<=> \(\frac{1}{x-2}=-\frac{1}{2x-1}\)

<=> x-2=1-2x <=> 3x=3

=> x=1

Đáp số: x=1

3 tháng 2 2019

Đặt \(B=1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{97}+\frac{1}{99}\)

\(=\left(1+\frac{1}{99}\right)+\left(\frac{1}{3}+\frac{1}{97}\right)+\left(\frac{1}{5}+\frac{1}{95}\right)+...+\left(\frac{1}{49}+\frac{1}{51}\right)\)

\(=\frac{100}{99}+\frac{100}{3\times97}+\frac{100}{5\times95}+...+\frac{100}{49\times51}\)

\(=100\left(\frac{1}{99}+\frac{1}{3\times97}+\frac{1}{5\times95}+...+\frac{1}{49\times51}\right)\)

Đặt \(C=\frac{1}{1\times99}+\frac{1}{3\times97}+\frac{1}{5\times95}+...+\frac{1}{97\times3}+\frac{1}{99\times1}\)

\(=2\left(\frac{1}{99}+\frac{1}{3\times97}+\frac{1}{5\times95}+...+\frac{1}{49\times51}\right)\)

\(A=\frac{B}{6}=\frac{100}{2}=50\)

Vậy \(A=50\)

5 tháng 10 2016

con gái hay con trai thế?

mk không bít nha

mk học lớp 7 thui

k nhé

thank nhìu

5 tháng 10 2016

ô điêu vậy li học lớp 9 rồi mà

24 tháng 2 2016

B= 2/3.5 + 2/5.7 +.....+ 2/37.39

  = 1/3 - 1/5 + 1/5 - 1/7 + ...... + 1/37 - 1/39

  = 1/3 - 1/39 =12/39

C = 1/4 - 1/7 + 1/7 - 1/10 + .....+ 1/73 - 1/ 76

  = 1/4 - 1/76 = 18/76

24 tháng 2 2016

B= 2/3.5 + 2/5.7 +.....+ 2/37.39

  = 1/3 - 1/5 + 1/5 - 1/7 + ...... + 1/37 - 1/39

  = 1/3 - 1/39 =12/39

C = 1/4 - 1/7 + 1/7 - 1/10 + .....+ 1/73 - 1/ 76

  = 1/4 - 1/76 = 18/76