(7 Ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$ has a focal distance of $2 c$, if the three numbers $a$, $b$, $c$ form a geometric sequence in order, then its eccentricity is $\qquad$ .
(7) Since $a^{2}=b^{2}+c^{2}, b^{2}=a c$, then $a^{2}=a c+c^{2}$, and $e=\frac{c}{a}$, so $e^{2}+e-1=0$,

its positive root $e=\frac{\sqrt{5}-1}{2}$.