\(a,\left(\sqrt{7}+\sqrt{5}\right)^2=12+2\sqrt{35}>12=\left(\sqrt{12}\right)^2\\ \Rightarrow\sqrt{7}+\sqrt{5}>\sqrt{12}\\ b,\left(\sqrt{8}+3\right)^2=14+12\sqrt{2};\left(6+\sqrt{2}\right)^2=10+12\sqrt{2};14+12\sqrt{2}>10+12\sqrt{2}\Rightarrow\sqrt{8}+3>6+\sqrt{2}\\ c,2=12-10;\left(5\sqrt{3}\right)^2=75< 100\\ \Rightarrow-5\sqrt{3}>-10\Rightarrow12-5\sqrt{3}>12-10=2\\ d,\left(\sqrt{27}+\sqrt{21}\right)^2=48+18\sqrt{7}>48=\left(\sqrt{48}\right)^2\\ \Rightarrow\sqrt{27}+\sqrt{21}>\sqrt{48}\)
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