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

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28.cos275°+cos215°+cos75°cos15°的值等于A.√6/2B.3/2C.5/4D.1+(√3/4)9.已知a·b=0,|a|=2,|b|=3,且(

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
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8.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.1
10.在△ABC中,已知tanA、tanB是方程3x2+8x-1=0的两个根,则tanC等于
A.2 B.-2 C.4 D.-4

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
8,[cos(75)]^2+[sin(75)]^2+cos(15)sin(15)=1+1/2sin(30)=1+1/4=5/4.c.5/4
9,0=(2a+2b)(ka-b)=2k|a|^2+ab(2k-2)-2|b|^2=4k-6,k=3/2.a.3/2
10,tan(A)+tan(B)=-8/3,tan(A)tan(B)=-1/3,tan(C)=tan(180-A-B)=-tan(A+B)=-[tan(A)+tan(B)]/[1-tan(A)tan(B)]=(8/3)/[1+1/3]=8/4=2