`a)` $\dfrac{1313}{2121}$ × $\dfrac{165165}{143143}$ × $\dfrac{424242}{151515}$

= $\dfrac{1313×165165×424242}{2121×143143×151515}$ 

= $\dfrac{13×101×165×1001×42×10101}{21×101×143×1001×15×10101}$ 

= $\dfrac{13×101×15×11×1001×21×2×10101}{21×101×13×11×1001×15×10101}$ 

= `2`

`b)` $\dfrac{1995}{1996}$ × $\dfrac{19961996}{19931993}$ × $\dfrac{199319931993}{199519951995}$

= $\dfrac{1995×19961996×199319931993}{1996×19931993×199519951995}$ 

= $\dfrac{1995×1996×10001×1993×100010001}{1996×1993×10001×1995×100010001}$ 

= `1`

Lời giải:

Bài 7:

`a, 1313/2121 xx (165165)/(143143) xx (424242)/(151515)`

  `= 13/21 xx 165/143 xx 42/15`

  `= (13 xx 165 xx 42)/(21 xx 143 xx 15)`

  `= (13 xx 11 xx 15 xx 2 xx 21)/(21 xx 11 xx 13 xx 15)`

  `= (\cancel{13} xx \cancel{11} xx \cancel{15} xx 2 xx \cancel{21})/(\cancel{21} xx \cancel{11} xx \cancel{13} xx \cancel{15})`

  `= 2`

`b, 1995/1996 xx (19961996)/(19931993) xx (199319931993)/(199519951995)`

  `= 1995/1996 xx 1996/1993 xx 1993/1995`

  `= (1995 xx 1996 xx 1993)/(1996 xx 1993 xx 1995)`

  `= 1`