D 解析：化简第一个式子（提公因式 利用平方差公式）得Sin(a3-a6)=1 得d=-π/6 带入前n项和公式得Sn=d/2n^2+(a1-d/2）n -b/2a为对称轴 对称轴应该大于等于8.5 小于等于9.5 解之得答案为d
1.设等差数列{an}满足:设等差数列{an}满足:[sin^2(a3)-cos^2(a3)+cos^2(a3)*cos
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