a) Cách1 :
\(\displaystyle{{ - 21} \over {28}} = {{ - 21:7} \over {28:7}} = {{ - 3} \over 4}\) \((1)\)
\(\displaystyle{{ - 39} \over {52}} = {{ - 39:13} \over {52:13}} = {{ - 3} \over 4}\) \((2)\)
Từ \((1)\) và \((2)\) ta suy ra: \(\displaystyle{{ - 21} \over {28}} = {{ - 39} \over {52}}.\)
Cách 2 : \(-21.52 = -1092\) ; \(28.(-39) = 1092\)
Suy ra: \(-21.52 = 28.(-39).\)
Vậy \(\displaystyle{{ - 21} \over {28}} = {{ - 39} \over {52}}.\)
b)
\(\displaystyle{{ - 1717} \over {2323}} = {{ - 1717:101} \over {2323:101}} = {{ - 17} \over {23}}\) \((3)\)
\(\displaystyle{{ - 171717} \over {232323}} = {{ - 171717:10101} \over {232323:10101}} \)\(\displaystyle= {{ - 17} \over {23}}\) \((4)\)
Từ \((3)\) và \((4)\) suy ra: \(\displaystyle{{ - 1717} \over {2323}} = {{ - 171717} \over {232323}}.\)
