Lời giải:
Vế trái luôn không âm (tính chất trị tuyệt đối)

$\Rightarrow -11x\geq 0$

$\Rightarrow x\leq 0$

Do đó: $x-\frac{1}{3}, x-\frac{1}{15},..., x-\frac{1}{399}<0$

PT trở thành:
$\frac{1}{3}-x+\frac{1}{15}-x+...+\frac{1}{399}-x=-11x$

$(\frac{1}{3}+\frac{1}{15}+...+\frac{1}{399})-10x=-11x$

$\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{19.21}=-x$

$\frac{1}{2}(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+..+\frac{1}{19}-\frac{1}{21})=-x$

$\frac{1}{2}(1-\frac{1}{21})=-x$

$\frac{10}{21}=-x$

$\Rightarrow x=\frac{-10}{21}$

Lời giải:
Vế trái luôn không âm (tính chất trị tuyệt đối)

$\Rightarrow -11x\geq 0$

$\Rightarrow x\leq 0$

Do đó: $x-\frac{1}{3}, x-\frac{1}{15},..., x-\frac{1}{399}<0$

PT trở thành:
$\frac{1}{3}-x+\frac{1}{15}-x+...+\frac{1}{399}-x=-11x$

$(\frac{1}{3}+\frac{1}{15}+...+\frac{1}{399})-10x=-11x$

$\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{19.21}=-x$

$\frac{1}{2}(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+..+\frac{1}{19}-\frac{1}{21})=-x$

$\frac{1}{2}(1-\frac{1}{21})=-x$

$\frac{10}{21}=-x$

$\Rightarrow x=\frac{-10}{21}$

a ) \(\left(x-1\right)\left(x+1\right)-x\left(x+1\right)=35\)

\(\Leftrightarrow x^2-1^2-x.x-x.1=35\)

\(\Leftrightarrow x^2-1^2-x^2-x=35\)

\(\Leftrightarrow-1-x=35\)

\(\Leftrightarrow-x=35+1\)

\(\Leftrightarrow-x=36\)

\(\Leftrightarrow x=-36\)

a) \(-2x\left(10x-3\right)+5x\left(4x+1\right)=25\)

\(-20x^2+6x+20x^2+5x=25\)

\(\Rightarrow6x+5x=25\)

\(\Rightarrow11x=25\)

\(\Rightarrow x=\dfrac{25}{11}\)

b) \(y\left(5-2y\right)+2y\left(y-1\right)=15\)

\(5y-2y^2+2y^2-2y=15\)

\(\Rightarrow5y-2y=15\)

\(\Rightarrow3y=15\)

\(\Rightarrow y=5\)

c)\(x\left(x+1\right)-\left(x+1\right)=35\)

\(\Rightarrow\left(x-1\right)\left(x+1\right)=35\)

\(\Rightarrow x^2-1=35\)

\(\Rightarrow x^2=36\)

\(\Rightarrow x=6;x=-6\)

d)\(x\left(x^2+x+1\right)-x^2\left(x+1\right)=0\)

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

\(\Rightarrow x^2+2x=0\)

\(\Rightarrow x\left(x+2\right)=0\)

\(\Rightarrow x=0;x=0-2=-2\)

Vậy \(x=0;x=-2\)

a: Sửa đề: \(\left(x-18\right)\left(x-7\right)\left(x+35\right)\left(x+90\right)-67x^2\)

\(=\left(x^2+17x-630\right)\left(x^2+83x-630\right)-67x^2\)

Đặt \(x^2+17x-630=a\)

Ta sẽ được \(a\left(a+66x\right)-67x^2\)

\(=a^2+66ax-67x^2\)

\(=\left(a+67x\right)\left(a-x\right)\)

\(=\left(x^2+84x-630\right)\left(x^2+16x-630\right)\)

b: \(\left(3x-2\right)^3+\left(1-2x\right)^3+\left(1-x\right)^3\)

Đặt 1-2x=a; 1-x=b

Ta sẽ được \(-\left(a+b\right)^3+a^3+b^3\)

\(=-a^3-b^3-3ab\left(a+b\right)+a^3+b^3\)

\(=-3ab\left(a+b\right)\)

\(=-\left(2-3x\right)\left(1-2x\right)\left(1-x\right)\)

a)Ta thấy:

\(\dfrac{1}{x}-\dfrac{1}{x+a}=\dfrac{x+a}{x\left(x+a\right)}-\dfrac{x}{x\left(x+a\right)}\)

\(=\dfrac{\left(x+a\right)-x}{x\left(x+a\right)}\)

\(=\dfrac{a}{x\left(x+a\right)}\)

\(\Rightarrowđpcm\)

b)Ta thấy:

\(\dfrac{1}{x\left(x+1\right)}-\dfrac{1}{\left(x+1\right)\left(x+2\right)}\)

\(=\dfrac{\left(x+1\right)\left(x+2\right)}{x\left(x+1\right)^2\left(x+2\right)}-\dfrac{x\left(x+1\right)}{x\left(x+1\right)^2\left(x+2\right)}\)

\(=\dfrac{x+2}{x\left(x+1\right)\left(x+2\right)}-\dfrac{x}{x\left(x+1\right)\left(x+2\right)}\)

\(=\dfrac{\left(x+2\right)-x}{x\left(x+1\right)\left(x+2\right)}=\dfrac{2}{x\left(x+1\right)\left(x+2\right)}\Rightarrowđpcm\)

c)Ta thấy:

\(\dfrac{1}{x\left(x+1\right)\left(x+2\right)}-\dfrac{1}{\left(x+1\right)\left(x+2\right)\left(x+3\right)}\)

\(=\dfrac{\left(x+1\right)\left(x+2\right)\left(x+3\right)}{x\left(x+1\right)^2\left(x+2\right)^2\left(x+3\right)}-\dfrac{x\left(x+1\right)\left(x+2\right)}{x\left(x+1\right)^2\left(x+2\right)^2\left(x+3\right)}=\dfrac{x+3}{x\left(x+1\right)\left(x+2\right)\left(x+3\right)}-\dfrac{x}{x\left(x+1\right)\left(x+2\right)\left(x+3\right)}=\dfrac{x+3-x}{x\left(x+1\right)\left(x+2\right)\left(x+3\right)}=\dfrac{3}{x\left(x+1\right)\left(x+2\right)\left(x+3\right)}\Rightarrowđpcm\)

a/ \(\dfrac{1}{x}-\dfrac{1}{x+a}=\dfrac{a}{x\left(x+a\right)}\)

Ta có: \(\dfrac{1}{x}-\dfrac{1}{x+a}=\dfrac{x+a}{x\left(x+a\right)}-\dfrac{x}{x\left(x+a\right)}\)

\(=\dfrac{\left(x-x\right)+a}{x\left(x+a\right)}\) hay \(\dfrac{a}{x\left(x+a\right)}\)

\(\Rightarrow\dfrac{1}{x}-\dfrac{1}{x+a}=\dfrac{a}{x\left(x+a\right)}\left(đpcm\right)\)

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

= (x-1)(5x+3)-(3x-8)(x-1)=0

=(x-1)[(5x+3)-(3x-8)]=0

=(x-1)(5x+3-3x+8)=0

=(x-1)(2x+11)=0

\(\Leftrightarrow\) x-1=0 hoặc 2x+11=0

\(\Leftrightarrow\) x=1 hoặc x=\(\dfrac{-11}{2}\)

Vậy S={1;\(\dfrac{-11}{2}\)}

b) 3x(25x+15)-35(5x+3)=0

=3x.5(5x+3)-35(5x+3)=0

=15x(5x+3)-35(5x+3)=0

=(5x+3)(15x-35)=0

\(\Leftrightarrow\) 5x+3=0 hoặc 15x-35=0

\(\Leftrightarrow\) x=\(\dfrac{-3}{5}\) hoặc x=\(\dfrac{7}{3}\)

Vậy S={\(\dfrac{-3}{5};\dfrac{7}{3}\)}

c) (2-3x)(x+11)=(3x-2)(2-5x)

=(2-3x)(x+11)-(3x-2)(2-5x)=0

=(3x-2)[(x+11)-(2-5x)]=0

=(3x-2)(x+11-2+5x)=0

=(3x-2)(6x+9)=0

\(\Leftrightarrow\) 3x-2=0 hoặc 6x+9=0

\(\Leftrightarrow\) x=\(\dfrac{2}{3}\) hoặc x=\(\dfrac{-3}{2}\)

Vậy S={\(\dfrac{2}{3};\dfrac{-3}{2}\)}

d) (2x²+1)(4x-3)=(2x²+1)(x-12)

=(2x²+1)(4x-3)-(2x²+1)(x-12)=0

=(2x²+1)[(4x-3)-(x-12)=0

=(2x²+1)(4x-3-x+12)=0

=(2x²+1)(3x+9)=0

\(\Leftrightarrow\)2x²+1=0 hoặc 3x+9=0

\(\Leftrightarrow\)x=\(\dfrac{1}{2}\)hoặc x=\(\dfrac{-1}{2}\) hoặc x=-3

Vậy S={\(\dfrac{1}{2};\dfrac{-1}{2};-3\)}

e) (2x-1)²+(2-x)(2x-1)=0

=(2x-1)[(2x-1)+(2-x)=0

=(2x-1)(2x-1+2-x)=0

=(2x-1)(x+1)=0

\(\Leftrightarrow\) 2x-1=0 hoặc x+1=0

\(\Leftrightarrow\) x=\(\dfrac{-1}{2}\) hoặc x=-1

Vậy S={\(\dfrac{-1}{2}\);-1}

f)(x+2)(3-4x)=x²+4x+4

=(x+2)(3-4x)=(x+2)²

=(x+2)(3-4x)-(x+2)²=0

=(x+2)[(3-4x)-(x+2)]=0

=(x+2)(3-4x-x-2)=0

=(x+2)(-5x+1)=0

\(\Leftrightarrow\) x+2=0 hoặc -5x+1=0

\(\Leftrightarrow\) x=-2 hoặc x=\(\dfrac{1}{5}\)

Vậy S={-2;\(\dfrac{1}{5}\)}