3.18. Find the eccentricity, coordinates of the foci, and vertices of the ellipse $x^{2} / 25+y^{2} / 9=1$.
Solution. According to the condition $a^{2}=25, b^{2}=9$, hence $a=5$, $b=3$. In accordance with the notations in Figure 3.10, we get $c=\sqrt{25-9}=4, \varepsilon=4 / 5=0.8, A_{1,2}(\mp 5 ; 0), B_{1,2}(0 ; \mp 3)$, $F_{1,2}(\mp 4 ; 0)$.