[tex]a_{15}=18\\a_{18}=-5\frac{1}{3}\\\\a_{18}=a_{15}*q^{18-15}\\a_{18}=a_{15}*q^3\\-5\frac{1}{3}=18*q^3\ |:18\\-\frac{16}{3}*\frac{1}{18}=q^3\\-\frac{8}{3}*\frac{1}{9}=q^3\\-\frac{8}{27}=q^3\\q=\sqrt[3]{-\frac{8}{27}}\\q=-\frac{2}{3}[/tex]
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[tex]a_{15}=18\\a_{18}=-5\frac{1}{3}\\\\a_{18}=a_{15}*q^{18-15}\\a_{18}=a_{15}*q^3\\-5\frac{1}{3}=18*q^3\ |:18\\-\frac{16}{3}*\frac{1}{18}=q^3\\-\frac{8}{3}*\frac{1}{9}=q^3\\-\frac{8}{27}=q^3\\q=\sqrt[3]{-\frac{8}{27}}\\q=-\frac{2}{3}[/tex]