a) =\(\frac{1}{2\cdot3}\)+\(\frac{1}{3\cdot4}\)+....+\(\frac{1}{10\cdot11}\)=\(\frac{1}{2}\)-\(\frac{1}{11}\)=\(\frac{9}{22}\)
b)\(\frac{2008}{2009}\)=1 - \(\frac{1}{2009}\); \(\frac{2007}{2008}\)=1 - \(\frac{1}{2008}\)Do \(\frac{1}{2009}\)<\(\frac{1}{2008}\)nen 1 - \(\frac{1}{2009}\)>1 - \(\frac{1}{2008}\)
=> 2008/2009>2007/2008
a) 1/2x3+1/3x4+1/4x5+...+1/10x11
=1/2-1/3+1/3-1/4+1/4-1/5+...+1/10-1/11
=1/2-1/11
=9/22
a)1/2x3 + 1/3x4 + 1/4x5 + .........+1/10x11
= 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ........+1/10 - 1/11
= 1/2 - 1/11
=9/22
b)1 - 2008/2009 = 1/2009 ; 1 - 2007/2008 = 1/2008
vì 1/2009 <1/2008
vậy 2008/2009>2007/2008
a. 1/6 + 1/12 + 1/20 + 1/30+ 1/42+1/56+1/72+1/90 +1/110
=1/2x3 +1/3x4 +1/4x5 +1/5x6 +1/6x7 +1/7x8 +1/8x9 +1/9x10+1/1x11
=1/2 -1/3 +1/3 -1/4 +1/4 - 1/5 +1/5 -1/6+1/6 - 1/7 +1/7 - 1/8 +1/8 -1/9 +1/9 -1/10 +1/10 - 1/11
gạch các phân số lặp lại , ta có:
= 1/2 -1/11
= 7/22
