²8.cos²75°+cos²15°+cos75°cos15°的值等于A.√6/2 B.3/2 C.5/4 D.1+（√3/4）9.已知a·b=0,|a|=2,|b|=3,且(2a+2b)·(ka-b)=0,则K的值为A.3/2 B.-3/2 C.±3/2 D.110.在△ABC中,已知tanA、tanB是方程3x²+8x-1=0的两个