CM: 1/4+1/9+1/16+...+1/10000<1
(1-1/4)*(1-1/9)*(1-1/16)*.......*(1-1/10000)=?
{1-1/4}×{1-1/9}×{1-1/16}×...×{1-1/10000}
\(\left(1-\frac{1}{4}\right).\left(1-\frac{1}{9}\right).\left(1-\frac{1}{16}\right)...\left(1-\frac{1}{10000}\right)\)
\(=\frac{3}{4}.\frac{8}{9}.\frac{15}{16}...\frac{9999}{10000}\)
\(=\frac{1.3}{2.2}.\frac{2.4}{3.3}.\frac{3.5}{4.4}...\frac{99.101}{100.100}\)
\(=\frac{1.2.3...99}{2.3.4...100}.\frac{3.4.5...101}{2.3.4...100}\)
\(=\frac{1}{100}.\frac{101}{2}=\frac{101}{200}\)
(1-1/4)(1-1/9)(1-1/16) .... (1-1/10000)
Tính:\(A=\left(1-\dfrac{1}{4}\right)\left(1-\dfrac{1}{9}\right)\left(1-\dfrac{1}{16}\right)...\left(1-\dfrac{1}{10000}\right)\)\(=\left(1-\dfrac{1}{2^2}\right)\left(1-\dfrac{1}{3^2}\right)\left(1-\dfrac{1}{4^2}\right)...\left(1-\dfrac{1}{100^2}\right)\)\(=\dfrac{3}{2^2}.\dfrac{2.4}{3^2}.\dfrac{3.5}{4^2}...\dfrac{99.101}{100^2}\)
\(=\dfrac{2.3.4...99}{2.3.4...100}.\dfrac{3.4.5.6...101}{2.3.4...100}\)
\(=\)\(\dfrac{1}{100}.\dfrac{101}{2}=\dfrac{101}{200}\)
S=1/4+1/9+1/16+...+1/10000
chứng tỏ S<1
S=1/4+1/9+1/16+...+1/10000 = 1/2x 2 + 1/3x3+...+1/100x100 < 1/1x2 + 1/2x3 +...+ 1/9x10 = 1 - 1/2 + 1/2 - 1/3 +...+ 1/9 - 1/10 = 1- 1/10 < 1
S=1/4+1/9+1/16+...+1/10000
chứng tỏ S<1
S=1/4+1/9+1/16+...+1/10000
= 1/2x 2 + 1/3x3+...+1/100x100 < 1/1x2 + 1/2x3 +...+ 1/9x10
= 1 - 1/2 + 1/2 - 1/3 +...+ 1/9 - 1/10 = 1- 1/10 < 1
So sánh A= 1/4+1/9+1/16+1/25+.....+1/10000 và 3/4
A=1/(2x2)+1/(3x3)+...+1/(100x100)
Nhận thấy rằng n x n -1=n x n -n+n-1=n x (n-1)+n-1=(n-1) x (n+1)
=> A < 1/(2x2-1)+1/(3x3-1)+...+1/(100x100-1)=1/(1x3)+1/(3x5)+...+1/(99x101)=1/2-1/202<1/2<3/4
so sánh A= 1/4+1/9+1/16+1/25+.....+1/10000 và 3/4
A=1/(2x2)+1/(3x3)+...+1/(100x100) Nhận thấy rằng n x n -1=n x n -n+n-1=n x (n-1)+n-1=(n-1) x (n+1) => A < 1/(2x2-1)+1/(3x3-1)+...+1/(100x100-1)=1/(1x3)+1/(3x5)+...+1/(99x101)=1/2-1/202<1/2<3/4
tính : A= (1-1/4) . (1-1/9) . (1-1/16) ...(1-1/10000)
So sánh K và 1
K=1/4+1/9+1/16+1/25+...+1/10000