C,C
ä¸é¢é£ä¸ªéA
å 为a7=a6+2a5 æ以a5q^2=a5q+2a5ï¼æ以q=2æq=-1
æ ¹å·aman=4a1 æä»¥æ ¹å·a1q^(m-1)*a1q^(n-1)=4a1 å¾m+n=6
æ以é¢ç®åç®ä¸ºm+n=6 ä¸ 1/m+4/nçæå°å¼
å 为m+n=6æ以(m+n)/6=1
æ以1/m+4/n = (1/m+4/n)(m/6+n/6) = 1/6+n/6m+4m/6n+4/6 = 5/6+n/6m+2m/3n
ç¨åå¼å®ç n/6m+2m/3n >= 2è·å·n/6m*2m/3n=2æ ¹å·1/9=2/3
æ以5/6+n/6m+2m/3n >= 2/3
æ以æå°å¼æ¯2/3
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