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![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{1}{1.\left(2n-1\right)}+\frac{1}{3.\left(2n-3\right)}+...+\frac{1}{\left(2n-3\right).3}+\frac{1}{\left(2n-1\right).1}\)
\(=\frac{1}{2n}\left[\frac{2n-1+1}{1\left(2n-1\right)}+\frac{2n-3+3}{3\left(2n-3\right)}+...+\frac{3+2n-3}{\left(2n-3\right).3}+\frac{1+2n-1}{\left(2n-1\right).1}\right]\)
\(=\frac{1}{2n}\left(1+\frac{1}{2n-1}+\frac{1}{3}+\frac{1}{2n-3}+...+\frac{1}{2n-3}+\frac{1}{3}+\frac{1}{2n-1}+1\right)\)
\(=\frac{1}{n}\left(1+\frac{1}{3}+...+\frac{1}{2n-3}+\frac{1}{2n-1}\right)\)
\(\Rightarrow\frac{A}{B}=\frac{1}{n}\).
![](https://rs.olm.vn/images/avt/0.png?1311)
\(P=\frac{n^3+2n^2-1}{n^3+2n^2+2n+1}\)
ĐKXĐ : \(n\ne-1\)
\(=\frac{n^3+n^2+n^2+n-n-1}{n^3+2n^2+2n+1}=\frac{n^2\left(n+1\right)+n\left(n+1\right)-\left(n+1\right)}{\left(n^3+1\right)+2n\left(n+1\right)}\)
\(=\frac{\left(n+1\right)\left(n^2+n-1\right)}{\left(n+1\right)\left(n^2-n+1\right)+2n\left(n+1\right)}=\frac{\left(n+1\right)\left(n^2+n-1\right)}{\left(n+1\right)\left(n^2+n+1\right)}=\frac{n^2+n-1}{n^2+n+1}\)
Với n nguyên, đặt ƯC( n2 + n - 1 ; n2 + n + 1 ) = d
=> n2 + n - 1 ⋮ d và n2 + n + 1 ⋮ d
=> ( n2 + n + 1 ) - ( n2 + n - 1 ) ⋮ d
=> n2 + n + 1 - n2 - n + 1 ⋮ d
=> 2 ⋮ d => d = 1 hoặc d = 2
Dễ thấy n2 + n + 1 ⋮/ 2 ∀ n ∈ Z ( bạn tự chứng minh )
=> loại d = 2
=> d = 1
=> ƯCLN( n2 + n - 1 ; n2 + n + 1 ) = 1
hay P tối giản ( đpcm )
![](https://rs.olm.vn/images/avt/0.png?1311)
1)5(x^2-1)+x(1-5x)= x-2
<=>5x2-5+x-5x2=x-2
<=>-5+x=x-2
<=>x-x=-2+5
<=>0x=3(vô lí)
vậy ko tìm được x
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
\(A=\frac{1}{1(2n-1)}+\frac{1}{3(2n-3)}+...+\frac{1}{(2n-3).3}+\frac{1}{(2n-1).1}\)
\(2nA=\frac{1+(2n-1)}{1(2n-1)}+\frac{3+(2n-3)}{3(2n-3)}+....+\frac{(2n-3)+3}{(2n-3).3}+\frac{(2n-1)+1}{(2n-1).1}\)
\(2nA=\frac{1}{2n-1}+1+\frac{1}{2n-3}+\frac{1}{3}+...+\frac{1}{3}+\frac{1}{2n-3}+1+\frac{1}{2n-1}\)
\(=\left(\frac{1}{2n-1}+\frac{1}{2n-3}+...+\frac{1}{3}+1\right)+\left(1+\frac{1}{3}+...+\frac{1}{2n-3}+\frac{1}{2n-1}\right)\)
\(=2\left(1+\frac{1}{3}+...+\frac{1}{2n-1}\right)\)
\(\Rightarrow A=\frac{1}{n}\left(1+\frac{1}{3}+...+\frac{1}{2n-1}\right)\)
\(A=\frac{1}{1\left(2n-1\right)}+\frac{1}{3\left(2n-3\right)}+...+\frac{1}{\left(2n-1\right).1}\)
\(A=\frac{1}{2n}\left[\frac{2n-1+1}{1\left(2n-1\right)}+\frac{2n-3+3}{3\left(2n-3\right)}+...+\frac{1+2n-1}{\left(2n-1\right).1}\right]\)
\(A=\frac{1}{2n}\left[\frac{1}{1}+\frac{1}{2n-1}+\frac{1}{3}+\frac{1}{2n-3}+...+\frac{1}{2n-1}+\frac{1}{1}\right]\)
\(A=\frac{1}{n}\left(1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{2n-3}+\frac{1}{2n-1}\right)\)
\(\Rightarrow\frac{a}{b}=\frac{1}{n}\).