3) Ta có

$1 \dfrac{1}{3} \times 1 \dfrac{1}{8} \times \dots \times 1 \dfrac{1}{9800} = \dfrac{4}{3} \times \dfrac{9}{8}  \times \dfrac{16}{15} \times \dots \times \dfrac{9801}{9800}$

$= \dfrac{2\times 2}{3 \times 1} \times \dfrac{3 \times 3}{4 \times 2} \times \dfrac{4 \times 4}{5 \times 3} \times \dots \times \dfrac{99\times 99}{100 \times 98}$

$= \dfrac{2\times 99}{100} = \dfrac{99}{50}$

4) Ta có

$\left( 1 - \dfrac{1}{5} \right) \times\left( 1 - \dfrac{1}{6} \right) \times\left( 1 - \dfrac{1}{7} \right) \times\left( 1 - \dfrac{1}{8} \right) \times\left( 1 - \dfrac{1}{9} \right) = \dfrac{4}{5} \times \dfrac{5}{6} \times \dfrac{6}{7} \times \dfrac{7}{8} \times \dfrac{8}{9} = \dfrac{4}{9}$

5) Ta có

$\left( 1 - \dfrac{1}{2} \right) \times\left( 1 - \dfrac{1}{3} \right) \times \dots \times \left( 1 - \dfrac{1}{2006} \right)= \dfrac{1}{2} \times \dfrac{2}{3} \times \dots \times \dfrac{2005}{2006} = \dfrac{1}{2006}$

Đáp án : 4) Ta có : 

( 1 - 1/5 ) x ( 1 - 1/6 ) x ( 1 - 1/7 ) x ( 1 - 1/8 ) x ( 1 - 1/9 ) 

= 4/5 x 5/6 x 6/7 x 7/8 x 8/9 

= 4/9

5) Ta có : 

( 1 - 1/2 ) x ( 1 - 1/3 ) x ... x ( 1 - 1/2006 ) 

= 1/2 x 2/3 x ..... x 2005/2006

= 1/2006

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