Răspuns :
[tex]\it \dfrac{12}{\sqrt3}+\dfrac{15}{5\sqrt3}-\dfrac{21}{7\sqrt3}+\dfrac{18}{\sqrt3}=\dfrac{1}{\sqrt3}(12+3-3+18)=\dfrac{^{\sqrt3)}30}{\ \sqrt3}=\dfrac{30\sqrt3}{3}=10\sqrt3[/tex]
[tex]\it \dfrac{12}{\sqrt3}+\dfrac{15}{5\sqrt3}-\dfrac{21}{7\sqrt3}+\dfrac{18}{\sqrt3}=\dfrac{1}{\sqrt3}(12+3-3+18)=\dfrac{^{\sqrt3)}30}{\ \sqrt3}=\dfrac{30\sqrt3}{3}=10\sqrt3[/tex]