S= - 3²\(\left(\frac{1}{4}+\frac{1}{28}+\frac{1}{70}+...+\frac{1}{868}\right)\)

S = - 3²\(\left(\frac{1}{1.4}+\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{28.31}\right)\)

S = - 3\(\left(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{28.31}\right)\)

S = -3\(\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{28}-\frac{1}{31}\right)\)

S = -3 \(\left(1-\frac{1}{31}\right)\)

S = -3\(.\frac{30}{31}\)

S = -90/31

1/3S=-(1/1*4+1/4*7+1/7*10+...+1/28*31)=-(1/1-1/4+1/4-1/7+1/7-1/10+...+1/28-1/31)=-(1/1-1/31)=-30/31

=>S=(-30/31):1/3=-90/31

a)    (2x + 1)(3x - 2) = (5x - 8)(2x + 1)

 <=> 6x² - x - 2 = 10x² - 11x - 8

<=>  6x² - 10x² - x + 11x -2 + 8 = 0

<=>  -4x² + 10x + 6  = 0

<=> -2 (2x² - 5x - 3) = 0

<=> 2x² - 5x - 3 = 0 

<=> 2x² - 6x + x - 3 = 0

<=> x (2x + 1) - 3 (2x + 1) = 0

<=> (x - 3) (2x + 1) = 0

* x - 3 = 0  => x = 3

* 2x + 1 = 0 => x = -1/2 

S = {-1/2; 3}

b) 4x² – 1 = (2x +1)(3x -5)

<=> 4x² – 1 - (2x +1)(3x -5) = 0

<=> (2x - 1) (2x + 1) - (2x + 1)(3x - 5) = 0

<=>  (2x + 1) (2x - 1 - 3x + 5) = 0

<=>  (2x + 1) (-x + 4) = 0

* 2x + 1 = 0  <=> x = -1/2

* -x + 4 = 0 <=> x = 4

S = {-1/2; 4}

c) (x + 1)² = 4(x² – 2x + 1)

<=> (x + 1)² - 4(x² – 2x + 1) = 0

<=> (x + 1)² - 4(x² – 1)² = 0

* (x + 1)² = 0   <=> x = -1

* 4(x² - 1)² = 0  <=> x = 1 và x = -1

S = {-1;  1}

d) 2x³ + 5x² – 3x = 0

<=> x (2x² + 5x - 3) = 0

<=> x (2x² + 6x - x - 3) = 0

<=> x [x(2x - 1) + 3 (2x - 1)] = 0

<=> x (2x - 1) (x + 3) = 0

* x = 0

* 2x - 1 = 0  <=> x = 1/2

* x + 3 = 0  <=> x = -3

S = { -3; 0; 1/2}

\(\frac{1}{x^2+5x+4}+\frac{1}{x^2+11x+28}+\frac{1}{x^2+17x+70}=\frac{3}{4x-2}\)

\(\Leftrightarrow\frac{1}{\left(x+1\right)\left(x+4\right)}+\frac{1}{\left(x+4\right)\left(x+7\right)}+\frac{1}{\left(x+7\right)\left(x+10\right)}=\frac{3}{4x-2}\)

\(\Leftrightarrow3x^2+21x+36=0\)

\(\Leftrightarrow x=-3\)

A = \(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{56}\)

\(=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{7.8}\)

  \(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{7}-\frac{1}{8}\)

  \(=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)

B = \(\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{11.13}\)

  \(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{11}-\frac{1}{13}\)

  \(=1-\frac{1}{13}=\frac{12}{13}\)

\(A=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{56}\)

\(=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{7.8}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{7}-\frac{1}{8}\)

\(=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)

\(B=\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{11.13}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{11}-\frac{1}{13}\)

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

A=\(\frac{1}{6}\)+\(\frac{1}{12}\)+......+\(\frac{1}{56}\)

A=\(\frac{1}{2.3}\)+\(\frac{1}{3.4}\)+...+\(\frac{1}{7.8}\)

A=\(\frac{1}{2}\)-\(\frac{1}{3}\)+\(\frac{1}{3}\)-\(\frac{1}{4}\)+....+\(\frac{1}{7}\)-\(\frac{1}{8}\)

A=\(\frac{1}{2}\)-\(\frac{1}{8}\)=\(\frac{3}{8}\)

B=\(\frac{2}{1.3}\)+\(\frac{2}{3.5}\)+...+\(\frac{2}{11.13}\)

B=1-\(\frac{1}{3}\)+\(\frac{1}{3}\)-\(\frac{1}{5}\)+....+\(\frac{1}{11}\)-\(\frac{1}{13}\)

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

C=\(\frac{3}{4}\)+\(\frac{3}{28}\)+....+\(\frac{3}{304}\)

C=\(\frac{3}{1.4}\)+\(\frac{3}{4.7}\)+..+\(\frac{3}{16.19}\)

Rồi bạn cũng tách ra như câu A và câu B

\(A=3\left(\frac{3}{1\cdot4}+\frac{3}{4\cdot7}+\frac{3}{7\cdot10}+.....+\frac{3}{55\cdot58}\right)\)

\(A=3\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+.....+\frac{1}{55}-\frac{1}{58}\right)\)

\(A=3\left(1-\frac{1}{58}\right)\)

\(A=3-\frac{1}{174}< 3< \frac{10}{3}\)

câu 2: \(S=\frac{25^{28^{ }}+25^{24}+...+25^2+25^2+1}{25^{28}.25^2+25^{24}.25^4+...+25^2+1}\)

         rút gọn ta được

          \(S=\frac{1}{25^4+1}\)

ko bit moi hoc lop 3

Giải Phương Trình Sau (Nhớ ghi cách làm nha mình k đúng cho)

Giải Phương Trình Sau (Nhớ ghi cách làm nha mình ĐÁNH DẤU ĐÚNG  cho)

bài 1+2: phân tích mẫu thành nhân tử r` áp dụng 

1/ab=1/a-1/b 

bài 3+4: quy đồng rút gọn blah...