Question

tính : B = 1/11 x 16 + 1 /16 x 21 + 1 /21 x 26 + .... 1 / 61 x 66

Answer 1

5B = 1/11 - 1/16 + 1/16 - 1/21 + 1/21 - 1/26 +.........+ 1/56 - 1/61 +1/61 - 1/66 
5B = 1/11 - 1/66 
5B = 5/66 
B = 1/66

Answer 2

1/66

nha bn

Answer 3

\(B=\frac{1}{11.16}+\frac{1}{16.21}+...+\frac{1}{61.66}\)

\(\Rightarrow5B=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)

\(\Rightarrow5B=\frac{6}{66}-\frac{1}{66}=\frac{5}{66}\)

\(\Rightarrow B=\frac{1}{66}\)