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因an是等比數列,所以a5=a2*q^3 162=6q^3 q=3 a1=a2/q=6/3=2 an=a1q^(n-1)=6*3^(n-1)=2*3^n sn=a1(1-q^n)/(1-q) 242=2*(1-3^n)/(1-3) n=5
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