Ja dota kāda racionāla izteiksme $$A$$, tad, pareizinot to ar $$-1$$, iegūst $\left(-1\right)\cdot A\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}-A$.
Divas racionālas izteiksmes $$A$$ un $$-A$$ sauc par savstarpēji pretējām racionālām izteiksmēm, ja to summa ir $$0$$, t.i., $$A+(-A)=0$$.
Tāpat kā savstarpēji pretēji skaitļi, arī divas savstarpēji pretējas izteiksmes viena no otras atšķiras tikai ar zīmi.

Pretēju izteiksmju piemēri:
$\begin{array}{l}1\right)\phantom{\rule{0.147em}{0ex}}5\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathrm{un}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}-5\\ 2\right)\phantom{\rule{0.147em}{0ex}}a+b\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathrm{un}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}-a-b\\ 3\right)\phantom{\rule{0.147em}{0ex}}\frac{x}{y}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathrm{un}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}-\frac{x}{y}\\ {4\right)\phantom{\rule{0.147em}{0ex}}m}^{2}-m+3\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\mathrm{un}\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}-{m}^{2}+m-3\end{array}$

Jo:
$\begin{array}{l}1\right)\phantom{\rule{0.147em}{0ex}}5+\left(-5\right)=5-5=0\\ \\ 2\right)\phantom{\rule{0.147em}{0ex}}\left(a+b\right)+\left(-a-b\right)=a+b-a-b=0\\ \\ 3\right)\phantom{\rule{0.147em}{0ex}}\frac{x}{y}+\left(-\frac{x}{y}\right)=\frac{x}{y}-\frac{x}{y}=0\\ \\ 4\right)\phantom{\rule{0.147em}{0ex}}\left({m}^{2}-m+3\right)+\left(-{m}^{2}+m-3\right)={m}^{2}-m+3-{m}^{2}+m-3=0\end{array}$

Izteiksmes $$m^2-m+3$$ un $$-m^2+m-3$$ ir savstarpēji pretēji polinomi.