$\sqrt[]{8-2\sqrt[]{7}}-\sqrt[]{8+2\sqrt[]{7}}$
=$\sqrt[]{(\sqrt[]{7})^2-2.\sqrt[]{7}.1+1^2}-\sqrt[]{(\sqrt[]{7})^2+2.\sqrt[]{7}.1+1^2}$
= $\sqrt[]{(\sqrt[]{7}-1)^2}-\sqrt[]{(\sqrt[]{7}+1)^2}$
=|$\sqrt[]{7}-1|-|\sqrt[]{7}+1|$
=$\sqrt[]{7}-1-(\sqrt[]{7}+1)$ (vì $\sqrt[]{7}-1$ > 0)
= $\sqrt[]{7}-1-\sqrt[]{7}-1$
= -2