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1. \(\cfrac{v-u}{u-v} =\) [ ].
2. \(\dfrac{(x-y)^2}{y^2-x^2} =\) [ ].
3. \(\dfrac{a-1}{1-a^2} =\) [ ].
4. \(\dfrac{m^3-m}{m-m^2} =\) [ ].