1.设等差数列{an}满足:[sin^2(a3)-cos^2(a3)+cos^2(a3)*cos^2(a6)-sin^2

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1.设等差数列{an}满足:[sin^2(a3)-cos^2(a3)+cos^2(a3)*cos^2(a6)-sin^2(a3)*sin^2(a6)]/sin(a4+a5)=1,公差d∈（-1,0）.若当且仅当n=9时,数列{an}的前n项和Sn取得最大值,则首项a1的取值范围是（）
A.(7π/6,4π/3) B.(4π/3,3π/2) C.[7π/6,4π/3] D.[4π/3,3π/2]
2.在△ABC中,角A、B、C的对边分别是a、b、c,则a>b是cosA

$1.设等差数列{an}满足:[sin^2(a3)-cos^2(a3)+cos^2(a3)*cos^2(a6)-sin^2$

由题意可知,概述列为（1）2（2）4（3）8……
（1）a3=[1+(cos π/2)^2]*a1+(sin π/2)^2=a1+1=2
a4=[1+(cos π)^2]*a2+(sin π)^2=2*a2=4
(2)Sk=a1+a3+……+a2k-1=1+2+……+k=1/2*(1+k)*k=1/2(k^2+k)
(3) n=2n-1（奇数）时 an=(n+1)/2
n=2n（偶数）时 an=2^(n/2)
可先做第三小问,判断出an,再算第二小问.

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