Question

[Cao Minh Hoàng]

E= 11/5.7+11/7.9+....+11/59.61

Answer 1

[Hồ Thu Giang]

E = \(\frac{11}{5.7}+\frac{11}{7.9}+...+\frac{11}{59.61}=11.\frac{1}{2}\left(\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{59.61}\right)\)

E = \(\frac{11}{2}.\left(\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{59}-\frac{1}{61}\right)\)

E = \(\frac{11}{2}.\left(\frac{1}{5}-\frac{1}{61}\right)=\frac{11}{2}.\frac{56}{305}\)

E = \(\frac{308}{305}\)

Answer 2

[Trần Đức Thắng]

E = 11/2 ( 2/5.7 + ... +2/59.61)

   = 11/2 ( 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/59 - 1/61)

   = 11/2 ( 1/5 - 1/61)

= 11/2 .56/305

 =308/305