[tex]\it a=\dfrac{8}{21}:2\dfrac{2}{7}+\dfrac{1}{3}=\dfrac{8}{21}:\dfrac{16}{7}+\dfrac{1}{3}=\dfrac{8}{21}\cdot\dfrac{7}{16}+\dfrac{1}{3}= \dfrac{1}{6}+\dfrac{^{2)}1}{\ 3}=\dfrac{\ 3^{(3}}{6}=\dfrac{1}{2}\\ \\ \\ Opusul\ lui\ a\ este\ -a\ =\ -\dfrac{1}{2}[/tex]
