Tính nhanh:{1+1/2}x{1+1/3}x{1+1/4}x...x{1+1/2015}=?
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=1/2 x 2/3 x 3/4 x 4/5 x 5/6 x.....x2013/2014 - 2014/2015
=1/2014 - 2014/2015
=1/2015
1) 9 x 10 + 10 x 11 + 11 x 12 + ....+ 2015 x 2016
=9x10x11-8x9x10+10x11x12-9x10x11+...+2015x2016x2017-2014x2015x2013
=2015x2016x2017-8x9x10
=8193537360
a.\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)...\left(1-\frac{1}{2015}\right)=\frac{1}{2}.\frac{2}{3}...\frac{2014}{2015}=\frac{1.2.3...2014}{2.3...2015}=\frac{1}{2015}\)
b.\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{8}+...+\frac{1}{128}-\frac{1}{256}=1-\frac{1}{256}=\frac{255}{256}\)
c.\(\frac{5}{2}+\frac{5}{4}+\frac{5}{8}+...+\frac{5}{256}=5\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\right)=5.\frac{255}{256}=\frac{1275}{256}\)
d.14,35+(13,7-13,6).1=14,35+0,1.1=14,35+0,1=14,45
( 2013 x 2014 +2014 x 2015 + 2015 x 2016 ) x ( 1 + 1/3 - 1 - 1/3 )
= ( 2013 x 2014 + 2014 x 2015 + 2015 x 2016 ) x 0
= 0
\(\frac{5932+6001x5931}{5932x6001-69}=\frac{6001x5931+5932}{5931x6001+6001-69}=\frac{6001x5931+5932}{6001x5931+5932}=1\)
(1-1/2)x(1-1/3)x(1-1/4)x.....x(1-1/2015)x(1-1/2016)
=> 1/2 x 2/3 x 3/4 x 4/5 x 5/6 x 6/7 x 7/8 x 8/9 x......... x 2014/2015 x 2015/2016
Ta rút gọn cho tử này mẫu kia còn: 1/2016.
Đáp số : 1/2016
\(=\frac{2-1}{2}\times\frac{3-1}{3}\times\frac{4-1}{4}\times...\times\frac{2015-1}{2015}\times\frac{2016-1}{2016}\)
\(=\frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times...\times\frac{2014}{2015}\times\frac{2015}{2016}\)
\(=\frac{1}{2016}\)
\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)..........\left(1-\frac{1}{2016}\right)\)
\(=\frac{1}{2}.\frac{2}{3}.........\frac{2015}{2016}\)
\(=\frac{1.2.......2015}{2.3.......2016}\)
\(=\frac{1}{2016}\)
\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)........\left(1-\frac{1}{2016}\right)\)
\(=\frac{1}{2}.\frac{2}{3}.......\frac{2015}{2016}=\frac{1}{2016}\)