Question

[nguyen ngoc trang]

tinh nhanh

a) 1/6 + 1/12 + 1/20 + ............ + 1/110

b) 2008/2009 va 2007/2008

Answer 1

[Do Thi Mai]

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

Answer 2

[Ko Có Tên]

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