[tex]a= \frac{4}{ \sqrt{5}+1 }= \frac{4( \sqrt{5} -1)}{( \sqrt{5}+1)( \sqrt{5}-1) }= \frac{4( \sqrt{5}-1) }{5-1}= \sqrt{5}-1 \\ b= \sqrt{15}: \sqrt{3}+1= \sqrt{ \frac{15}{3} }+1= \sqrt{5} +1 \\ M_{g}= \sqrt{ab}= \sqrt{( \sqrt{5}+1)( \sqrt{5}-1) }= \sqrt{5-1} = \sqrt{4}=2 [/tex]
