tính 3/4 . 8/9 . 15/16 . .... 9999/10000
tính A=3/4*8/9*15/16*....*9999/10000
\(A=\frac{3}{4}.\frac{8}{9}.\frac{15}{16}....\frac{9999}{10000}\)
\(=\frac{1.3}{2.2}.\frac{2.4}{3.3}.\frac{3.5}{4.4}.....\frac{99.101}{100.100}\)
\(=\frac{\left(1.2.3....99\right)\left(3.4.5....101\right)}{\left(2.3.4...100\right)\left(2.3.4...100\right)}\)
\(=\frac{1.101}{100.2}=\frac{101}{200}\)
Tính
3/4 . 8/9 . 15/16 ... 9999/10000
3/4 . 8/9 . 15/16 ... 9999/10000
= 1.3/2.2 . 2.4/3.3 ... 99.101/100.100
= 1 . 2 . ... . 99 / 2 . 3 . 100 × 3 . 4 ... 101 / 2 . 3 ... 100
= 1 / 100 . 101 / 2
= 101 / 200
=1.3/2.2 .2.4/3.3 .3.5/4.4 . ...... 99.101/100.100
=1.2.3.4.5 ...... .99/2.3.4.....100 . 3.4.5 ....... .101/2.3.4.5 .... .100
=1/100 .101/2
=101/200
k cho mink nha
\(\frac{3}{4}.\frac{8}{9}.\frac{15}{16}...\frac{9999}{10000}\)
\(=\frac{3.8.15...9999}{4.9.16...10000}\)
\(=\frac{\left(1.3\right).\left(2.4\right).\left(3.5\right)...\left(99.101\right)}{\left(2.2\right).\left(3.3\right).\left(4.4\right)...\left(100.100\right)}\)
\(=\frac{\left(1.2.3...99\right).\left(3.4.5...101\right)}{\left(2.3.4...100\right).\left(2.3.4...100\right)}\)
\(=\frac{1.101}{100.2}\)
\(=\frac{101}{200}\)
3/4 . 8/9 . 15/16 ... 9999/10000
3/4.8/9.15/16...9999/10000
=\(\dfrac{1.3}{2.2}\).\(\dfrac{2.4}{3.3}\)...\(\dfrac{99.101}{100.100}\)
=\(\dfrac{1.2...99}{2.3.100}\).\(\dfrac{3.4...101}{2.3.100}\)
=\(\dfrac{1}{100}\).\(\dfrac{101}{2}\)
=\(\dfrac{101}{200}\)
3/4+8/9+15/16+...+9999/10000=
3/4+8/9+15/16+...+9999//10000
\(\frac{3}{4}+\frac{8}{9}+\frac{15}{16}\)+...+\(\frac{9999}{10000}\)
= (1-\(\frac{1}{4}\)) +(1-\(\frac{1}{9}\))+(1-\(\frac{1}{16}\))+...+(1-\(\frac{1}{10000}\))
= 99 - (\(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}\)+....+\(\frac{1}{100^2}\)) => 99 - A
Dễ thấy A>0 =>S < 99 (1)
Lại có A= \(\frac{1}{2^2}\)+\(\frac{1}{3^2}\)+\(\frac{1}{4^2}\)+...+\(\frac{1}{100^2}\)
=> A<\(\frac{1}{1.2}\)+\(\frac{1}{2.3}\)+\(\frac{1}{3.4}\)+...+\(\frac{1}{99.100}\)
=>A<1-\(\frac{1}{2}\)+\(\frac{1}{2}\)-\(\frac{1}{3}\)+\(\frac{1}{3}\)-\(\frac{1}{4}\)+...\(\frac{1}{99}\)-\(\frac{1}{100}\)
=>A<1-\(\frac{1}{100}\)<1
...
Tính x=\(\frac{3}{4}+\frac{8}{9}+\frac{15}{16}+...+\frac{9999}{10000}\)
\(x=\frac{3}{4}+\frac{8}{9}+\frac{15}{16}+...+\frac{9999}{10000}\)
\(x=\frac{1.3}{2.2}+\frac{2.4}{3.3}+\frac{3.5}{4.4}+...+\frac{99.101}{100.100}\)
\(x=\frac{1.2...99}{2.3...100}.\frac{3.4...101}{2.3...100}\)
\(x=\frac{1}{100}.\frac{101}{2}\)
\(x=\frac{101}{200}\)
\(X=\frac{1.3}{2.2}+\frac{2.4}{3.3}+\frac{3.5}{4.4}+...+\frac{99.101}{100.100}\)
\(X=\frac{1.2.3....99}{2.3.4....100}.\frac{3.4.5....101}{2.3.4....100}\)
\(X=\frac{1}{100}.\frac{101}{2}\)
\(X=\frac{101}{200}\)
Study well
\(\dfrac{3}{4}.\dfrac{8}{9}.\dfrac{15}{16}.....\dfrac{9999}{10000}\)
\(=\dfrac{1.3}{2^2}.\dfrac{2.4}{3^2}.\dfrac{3.5}{4^2}...\dfrac{99.101}{100^2}\)
\(=\dfrac{1.2...99}{2.3...100}.\dfrac{3.4...101}{2.3...100}=\dfrac{1}{100}.\dfrac{101}{2}=\dfrac{101}{200}\)
3/4 x 8/9 x 15/16 x 24/25 x .... x 9999/10000
Tính nhanh
cm 3/4+8/9+15/16+......+9999/10000<99