a=(√7+2)^-1+(√7-2)^-1
[tex]a = \frac{1}{ \sqrt{7 + 2} } + \frac{1}{ \sqrt{7 - 2} } \\ a = \frac{ \sqrt{7 - 2} }{3} + \frac{ \sqrt{7 + 2} }{3} \\ a = \frac{ \sqrt{7} - 2 + \sqrt{7} + 2 }{3} \\ a = \frac{ \sqrt{7} + \sqrt{7} }{3} = \frac{2 \sqrt{7} }{3} [/tex]
