\(\left(1-\frac{1}{3}\right)\left(1-\frac{1}{6}\right)\left(1-\frac{1}{10}\right)......\left(1-\frac{1}{780}\right)\)
\(=\frac{2}{3}.\frac{5}{6}.\frac{9}{10}......\frac{779}{780}=\frac{4}{6}.\frac{10}{12}.\frac{18}{20}.....\frac{1558}{1560}\)
\(=\frac{1.4}{2.3}.\frac{2.5}{3.4}.\frac{3.6}{4.5}.....\frac{38.41}{39.40}\)
\(=\frac{\left(1.2.3.....38\right)\left(4.5.6.....41\right)}{\left(2.3.4.....39\right)\left(3.4.5.....40\right)}=\frac{1.41}{39.3}=\frac{41}{117}\)
(1-1/3)*(1-1/6)*(1-1/10)*(1-1/15)*...*(1-780)
tính tổng (1-1/3)(1-1/6)(1-1/10)(1-1/15). ... .(1-1/780)
tính
B=(1-1/3).( 1-1/6).(1-1/10).(1-1/15)...(1-1/780)
Bài 2 : TÍnh : D = (1 - 1/3) . (1 - 1/6 ). ( 1 - 1/10) . (1-1/15) . ... . ( 1 - 1/780)
B=(1-\(\dfrac{1}{3}\)) ✖(1-\(\dfrac{1}{6}\))✖(1-\(\dfrac{1}{10}\))✖(1-\(\dfrac{1}{15}\))✖....✖(1-\(\dfrac{1}{780}\))
A bằng {1-1/3} nhân {1-1/6} nhân {1-1/10} nhân {1-1/15} nhân v.v nhân {1-1/780}
\(\left(1-\dfrac{1}{3}\right)\left(1-\dfrac{1}{6}\right)\left(1-\dfrac{1}{10}\right)\left(1-\dfrac{1}{15}\right)...\left(1-\dfrac{1}{780}\right)\)
B=[1-1/3].[1-1/6].[1-1/10].[1-1/15].[1-1/21].......[1-1/780]
Tính nhanh
a,8/9.15/16.24/25.....2499/2500
b,(1-1/3)(1-1/6)(1-1/10)(1-1/15).....(1-1/780)
