$$\dfrac{\sqrt{4+\sqrt{7}}+\sqrt{4-\sqrt{7}}}{2}=\dfrac{\sqrt{8+2\sqrt{7}}+\sqrt{8-2\sqrt{7}}}{2\sqrt{2}}$$
$$=$$
$$\dfrac{\sqrt{(\sqrt{7}+1)^2}+\sqrt{(\sqrt{7}-1)^2}}{2\sqrt{2}}$$
$$=$$
$$\dfrac{{(\sqrt{7}+1)}+{(\sqrt{7}-1)}}{2\sqrt{2}}$$
$$=$$
$$\dfrac{\sqrt{14}}{2}$$