[질문] 삼차방정식 x세제곱-3x제곱+ax+8=0 의세근이등비수열일때 상수a의값은?

a,ar,ar제곱.근의합,곱공식.a+ar+ar제곱=3,세근의곱ar세제곱=-8.ar=-2,a=-r분의2.위식에대입,-r분의2(r제곱+r+1)=3.양변에r곱해서정리r=-2또는-분의1.a=1또는4