a) = \(\dfrac{12}{17}\)

b) = \(\dfrac{31}{37}\)

\(A=\frac{16-11}{11.16}+\frac{21-16}{16.21}+...+\frac{66-61}{61.66}\)

\(A=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}=\frac{1}{11}-\frac{1}{66}=\frac{5}{66}\)

Ta có: \(D=\frac{\frac{15}{6x16}+\frac{15}{16x26}+\frac{15}{26x36}}{\frac{33}{6x16}-\frac{63}{16x26}+\frac{93}{26x36}}\)

\(\Rightarrow D=\frac{15.\frac{1}{6x16}+15.\frac{1}{16x26}+15.\frac{1}{26x36}}{3.11.\frac{1}{6x16}-3.21.\frac{1}{16x26}+3.31.\frac{1}{26x36}}\)

\(\Rightarrow D=\frac{15.\left(\frac{1}{6x16}+\frac{1}{16x26}+\frac{1}{26x36}\right)}{3.\left(\frac{11}{6x16}-\frac{21}{16x26}+\frac{31}{26x36}\right)}\)

\(\Rightarrow D=5.\left(\frac{1}{6x16}+\frac{1}{16x26}+\frac{1}{26x36}\right):\left(\frac{11}{6x16}-\frac{21}{16x26}+\frac{31}{26x36}\right)\)

\(\frac{1}{11\times16}+\frac{1}{16\times21}+\frac{1}{21\times26}+...+\frac{1}{56\times61}+\frac{1}{61\times66}\)

\(=\frac{1}{5}\times\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+...+\frac{1}{56}-\frac{1}{61}+\frac{1}{61}-\frac{1}{66}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{11}-\frac{1}{66}\right)\)

\(=\frac{1}{5}\times\frac{5}{66}\)

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

\(\frac{1}{11\times16}+\frac{1}{16\times21}+\frac{1}{21\times26}+...+\frac{1}{56\times61}+\frac{1}{61\times66}\)

\(=\frac{1}{5}\times\left(\frac{5}{11\times16}+\frac{5}{16\times21}+\frac{5}{21\times26}+...+\frac{5}{56\times61}+\frac{5}{61\times66}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+...+\frac{1}{56}-\frac{1}{61}+\frac{1}{61}-\frac{1}{66}\right)\)

\(=\frac{1}{5}\times\left(\frac{1}{11}-\frac{1}{66}\right)\)

\(=\frac{1}{5}\times\frac{5}{66}=\frac{1}{66}\)

\(\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}+...+\frac{1}{61.66}\)

\(=\frac{1}{5}.\left(\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\right)\)

\(=\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)

\(=\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{66}\right)\)

\(=\frac{1}{5}.\frac{5}{66}\)

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

đặt A= dãy số trên.Ta có:

5A= \(\frac{5}{11x16}+\frac{5}{16x21}+...+\frac{5}{61x66}\)

=> 5A= \(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)

=> 5A = \(\frac{1}{11}-\frac{1}{66}\)

=> 5A= \(\frac{5}{66}\)

=> A=\(\frac{1}{66}\)

\(=\frac{1}{5}\left(\frac{5}{11.16}\frac{5}{16.21}\frac{5}{21.26}+......+\frac{5}{61.66}\right)\)

\(=\frac{1}{5}\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+....+\frac{1}{61}+\frac{1}{66}\right)\)

=\(\frac{1}{5}\left(\frac{1}{11}+\frac{1}{66}\right)\)

\(=\frac{1}{5}.\frac{7}{66}\)

\(=\frac{7}{330}\)

\(M=\frac{7}{1.6}+\frac{7}{6.11}+\frac{7}{11.16}+...+\frac{7}{256.261}\)

\(M=7\left(\frac{1}{1.6}+\frac{1}{6.11}+...+\frac{1}{256.261}\right)\)

\(M=7.\frac{1}{5}\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{256}-\frac{1}{261}\right)\)

\(M=\frac{7}{5}\left(1-\frac{1}{261}\right)\)

\(M=\frac{7}{5}.\frac{260}{261}\)

\(M=\frac{364}{261}\)

nhân 7/5vaof hai vế của M,ta có\(M\frac{5}{ }=\frac{ }{5}.\left(M\right)\\ thành\frac{5}{1.6}+...+\frac{5}{256.261}\\ \)kết quả là52/261

du em chiu 

`@` `\text {Ans}`

`\downarrow`

\(\text{ A = }\dfrac{1}{4\times8}+\dfrac{1}{8\times12}+\dfrac{1}{12\times16}+...+\dfrac{1}{172\times176}\)

\(\text{A = }\dfrac{1}{4}\times\left(\dfrac{4}{4\times8}+\dfrac{4}{12\times16}+...+\dfrac{4}{172\times176}\right)\)

\(\text{A = }\dfrac{1}{4}\times\left(\dfrac{1}{4}-\dfrac{1}{8}+\dfrac{1}{12}-\dfrac{1}{16}+...+\dfrac{1}{172}-\dfrac{1}{176}\right)\)

\(\text{A = }\dfrac{1}{4}\times\left(\dfrac{1}{4}-\dfrac{1}{176}\right)\)

\(\text{A = }\dfrac{1}{4}\times\dfrac{43}{176}\)

\(\text{A = }\dfrac{43}{704}\)

Đáp số: `\text {A =} 43/704.`

A = 1/4 x 8 + 1/8 x 12 + 1/12 x 16 + ... + 1/176 x 180

=> 4A = 4/4 x 8 + 4/8 x 12 + 4/12 x 16 + ... + 4/176 x 180

=> 4A = 1/4 - 1/8 + 1/8 - 1/12 + 1/12 - 1/16 + ... 1/176 - 1/180

=> 4A = 1/4 - 1/180

=> 4A = 45/180 - 1/180

=> 4A = 44/180

=> 4A = 11/45

=> A = 11/45 : 4

=> A  = 11/180

Vậy A = 11/180

A = \(\dfrac{1}{4\times8}\) + \(\dfrac{1}{8\times12}\) + \(\dfrac{1}{12\times16}\) +...+ \(\dfrac{1}{176\times180}\)

A = \(\dfrac{1}{4}\) \(\times\)( \(\dfrac{4}{4\times8}\)+ \(\dfrac{4}{12\times16}\)+...+ \(\dfrac{4}{176\times180}\))

A = \(\dfrac{1}{4}\) \(\times\)( \(\dfrac{1}{4}\) - \(\dfrac{1}{8}\) + \(\dfrac{1}{12}\) - \(\dfrac{1}{16}\) +...+ \(\dfrac{1}{176}\) - \(\dfrac{1}{180}\))

A = \(\dfrac{1}{4}\) \(\times\)(\(\dfrac{1}{4}\) - \(\dfrac{1}{180}\))

A = \(\dfrac{1}{4}\) \(\times\)\(\dfrac{11}{45}\)

A = \(\dfrac{11}{180}\)