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^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]