Lời giải:

\(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{1}{x+y+z}\)

\(\Leftrightarrow \frac{1}{x}+\frac{1}{y}+\frac{1}{z}-\frac{1}{x+y+z}=0\)

\(\Leftrightarrow \frac{x+y}{xy}+\frac{x+y}{z(x+y+z)}=0\)

\(\Leftrightarrow (x+y)\left(\frac{1}{xy}+\frac{1}{z(x+y+z)}\right)=0\)

\(\Leftrightarrow (x+y).\frac{z(x+y+z)+xy}{xyz(x+y+z)}=0\)

\(\Leftrightarrow (x+y).\frac{z(y+z)+x(z+y)}{xyz(x+y+z)}=0\)

\(\Leftrightarrow \frac{(x+y)(z+x)(z+y)}{xyz(x+y+z)}=0\Rightarrow (x+y)(y+z)(x+z)=0\)

\(\Rightarrow \left[\begin{matrix} x=-y\\ y=-z\\ z=-x\end{matrix}\right.\)

Không mất tổng quát, giả sử \(x=-y\):

\(\frac{1}{x^{2003}}+\frac{1}{y^{2003}}+\frac{1}{z^{2003}}=\frac{1}{(-y)^{2003}}+\frac{1}{y^{2003}}+\frac{1}{z^{2003}}=\frac{1}{z^{2003}}\)

\(\frac{1}{x^{2003}+y^{2003}+z^{2003}}=\frac{1}{(-y)^{2003}+y^{2003}+z^{2003}}=\frac{1}{z^{2003}}\)

Do đó: \(\frac{1}{x^{2003}}+\frac{1}{y^{2003}}+\frac{1}{z^{2003}}=\frac{1}{x^{2003}+y^{2003}+z^{2003}}\) (đpcm)

\(\dfrac{x^2+y^2+z^2}{a^2+b^2+c^2}=\dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}+\dfrac{z^2}{c^2}\)

=\(\left(\dfrac{x^2}{a^2}-\dfrac{x^2}{a^2+b^2+c^2}\right)+\left(\dfrac{y^2}{b^2}-\dfrac{y^2}{a^2+b^2+c^2}\right)\)+\(\left(\dfrac{z^2}{c^2}-\dfrac{z^2}{a^2+b^2+c^2}\right)=0\)

=\(x^2.\dfrac{b^2+c^2}{a^2+b^2+c^2}+y^2.\dfrac{a^2+c^2}{a^2+b^2+c^2}+z^2.\dfrac{a^2+b^2}{a^2+b^2+c^2}=0\)

Vì \(a,b,c\) \(\ne\)0 nên dấu "=" xảy ra khi \(x=y=z=0\)

\( \Rightarrow\)\(A=x^{2003}+y^{2003}+z^{2003}=0+0+0=0\)

Chúc Bạn Học Tốt !!!

Ta có: \(2003^{2003}+1=2003^{2002+1}+1và2003^{2004}+1=2003^{2003+1}+1\)

\(\Rightarrow A>B\)

A > B

`(2-x)/2002-1=(1-x)/2003-x/2004`

`<=>(2-x)/2002-1+(x-1)/2003+x/2004=0`(chuyển  vế)

`<=>(2-x)/2002+1+(x-1)/2003-1+x/2004-1=0`

`<=>(2004-x)/2002+(x-2004)/2003+(x-2004)/2004=0`

`<=>(x-2004)(1/2003+1/2004-1/2002)=0`

`<=>x=2004` do `1/2003+1/2004-1/2002 ne 0`

Vậy `x=2004`

=>\(\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2001}{2003}\)

=>\(\dfrac{1}{6}+\dfrac{1}{12}+...+\dfrac{1}{x\left(x+1\right)}=\dfrac{2001}{4006}\)

=>\(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{2001}{4006}\)

=>\(\dfrac{1}{2}-\dfrac{1}{x+1}=\dfrac{2001}{4006}\)

=>1/(x+1)=1/2-2001/4006=1/2003

=>x+1=2003

=>x=2002

Bài 1.

Giải

a) Ta có: \(A=\dfrac{3n+9}{n-4}=\dfrac{3n-12+21}{n-4}=\dfrac{3\left(n-4\right)+21}{n-4}=3+\dfrac{21}{n-4}\)

Để \(A\in Z\) thì \(\dfrac{21}{n-4}\in Z\)

\(\Rightarrow21⋮\left(n-4\right)\)

\(\Rightarrow\left(n-4\right)\inƯ\left(21\right)\)

\(\Rightarrow\left(n-4\right)\in\left\{\pm1;\pm3;\pm7;\pm21\right\}\)

Ta có bẳng sau:

Vậy \(n\in\left\{-17;-3;1;3;5;7;11;25\right\}\) thì \(A\in Z.\)

b) Ta có: \(B=\dfrac{6n+5}{2n-1}=\dfrac{6n-3+8}{2n-1}=\dfrac{3\left(2n-1\right)+8}{2n-1}=3+\dfrac{8}{2n-1}\)

Để \(B\in Z\) thì \(\dfrac{8}{2n-1}\in Z\)

\(\Rightarrow8⋮\left(2n-1\right)\)

\(\Rightarrow\left(2n-1\right)\inƯ\left(8\right)\)

\(\Rightarrow\left(2n-1\right)\in\left\{\pm1;\pm2;\pm4;\pm8\right\}\)

Ta có bảng sau:

Vậy \(n\in\left\{\dfrac{-7}{2};\dfrac{-3}{2};\dfrac{-1}{2};0;1;\dfrac{3}{2};\dfrac{5}{2};\dfrac{9}{2}\right\}\)

Bạn Nguyen Thi Huyen giải bài 1 rồi nên mình giải tiếp các bài kia nhé!

Bài 2:

\(\dfrac{x-18}{2000}+\dfrac{x-17}{2001}=\dfrac{x-16}{2002}+\dfrac{x-15}{2003}\)

\(\Leftrightarrow\left(\dfrac{x-18}{2000}-1\right)+\left(\dfrac{x-17}{2001}-1\right)=\left(\dfrac{x-16}{2002}-1\right)+\left(\dfrac{x-15}{2003}-1\right)\)

\(\Leftrightarrow\dfrac{x-2018}{2000}+\dfrac{x-2018}{2001}=\dfrac{x-2018}{2002}+\dfrac{x-2018}{2003}\)

\(\Leftrightarrow\dfrac{x-2018}{2000}+\dfrac{x-2018}{2001}-\dfrac{x-2018}{2002}-\dfrac{x-2018}{2003}=0\)

\(\Leftrightarrow\left(x-2018\right)\left(\dfrac{1}{2000}+\dfrac{1}{2001}-\dfrac{1}{2002}-\dfrac{1}{2003}\right)=0\)

Dễ thấy \(\dfrac{1}{2000}>\dfrac{1}{2001}>\dfrac{1}{2002}>\dfrac{1}{2003}\) nên:

\(\dfrac{1}{2000}+\dfrac{1}{2001}+\dfrac{1}{2002}+\dfrac{1}{2003}\ne0\). Do đó:

\(x-2018=0\Leftrightarrow x=2018\)

Bài 3:

a) \(\dfrac{5}{x}+\dfrac{y}{4}=\dfrac{1}{8}\Leftrightarrow\dfrac{20}{4x}+\dfrac{xy}{4x}=\dfrac{20+xy}{4x+4x}=\dfrac{20+xy}{8x}=\dfrac{1}{8}\)

Hoán vị ngoại tỉ ta có: \(\dfrac{20+xy}{8x}=\dfrac{1}{8}\Leftrightarrow\dfrac{8}{8x}=\dfrac{1}{x}=\dfrac{1}{8}\Leftrightarrow x=8\)

Thế x = 8 vào : \(\dfrac{5}{x}+\dfrac{y}{4}=\dfrac{1}{8}\) .Ta có: \(\dfrac{5}{8}+\dfrac{y}{4}=\dfrac{1}{8}\Leftrightarrow\dfrac{y}{4}=\dfrac{1}{8}-\dfrac{5}{8}=\dfrac{-2}{4}\). Ta có: \(\dfrac{y}{4}=\dfrac{-2}{4}\Leftrightarrow y=-2\)

Vậy: \(\left[{}\begin{matrix}x=8\\y=-2\end{matrix}\right.\)

b) \(\dfrac{1}{x}-\dfrac{2}{y}=\dfrac{3}{1}\Rightarrow\dfrac{y}{x}-2=\dfrac{3}{1}\) (hoán vị ngoại tỉ)

\(\Leftrightarrow\dfrac{y}{x}=\dfrac{5}{1}\). Suy ra nghiệm x,y có dạng \(\left[{}\begin{matrix}x=1k\\y=5k\end{matrix}\right.\left(k\in Z\right)\). Bằng các phép thử lại ta dễ dàng suy ra x,y vô nghiệm.

\(\Leftrightarrow\dfrac{x+1}{2004}+1+\dfrac{x+2}{2003}+1=\dfrac{x+3}{2002}+1+\dfrac{x+4}{2001}+1\)

\(\Leftrightarrow\left(x+2005\right)\left(\dfrac{1}{2004}+\dfrac{1}{2003}-\dfrac{1}{2002}-\dfrac{1}{2001}\right)=0\)

\(\Leftrightarrow x=-2005\)

\(\dfrac{x+4}{2000}+\dfrac{x+3}{2001}=\dfrac{x+2}{2002}+\dfrac{x+1}{2003}=0\)

<=> \(\left(\dfrac{x+4}{2000}+1\right)+\left(\dfrac{x+3}{2001}+1\right)=\left(\dfrac{x+2}{2002}+1\right)+\left(\dfrac{x+1}{2003}+1\right)=0\)

<=> \(\dfrac{x+2004}{2000}+\dfrac{x+2004}{2001}=\dfrac{x+2004}{2002}+\dfrac{x+2004}{2003}\)

<=> \(\left(x+2004\right)\left(\dfrac{1}{2000}+\dfrac{1}{2001}-\dfrac{1}{2002}-\dfrac{1}{2003}\right)=0\)

<=> x + 2004 = 0

<=> x = -2004

(Bn nhớ thêm kết quả là 0 vào sau nữa nha)

Mik sửa:

\(\dfrac{x+4}{2000}+\dfrac{x+3}{2001}=\dfrac{x+2}{2002}+\dfrac{x+1}{2003}\)

<=> \(\left(\dfrac{x+4}{2000}+1\right)+\left(\dfrac{x+3}{2001}+1\right)=\left(\dfrac{x+2}{2002}+1\right)+\left(\dfrac{x+1}{2003}+1\right)\)

<=> \(\dfrac{x+2004}{2000}+\dfrac{x+2004}{2001}-\dfrac{x+2004}{2002}-\dfrac{x+2004}{2003}=0\)

<=> \(\left(x+2004\right)\left(\dfrac{1}{2000}+\dfrac{1}{2001}-\dfrac{1}{2002}-\dfrac{1}{2003}\right)=0\)

<=> x + 2004 = 0

<=> x = -2004