1.sin6°cos24°sin78°cos48°= sin6°cos24°cos12°cos48°cos6°sin6°cos24°cos12°cos48°/cos6°=(1/16)sin 96°=(1/16)sin 84°
3.cos^2 10°+cos^2 110°+cos^2 130°= cos^2 10°+cos^2 70°+cos^2 50°=sin^2 80°+sin^2 20°+sin^2 40°
cos 2a=1-2sin^2 a
4.cos2π/7cos4π/7cos6π/7=sin2π/7cos2π/7cos4π/7cos6π/7=(-1/8)/sin2π/7
6.(sina-sinb)^2=(4/7)-(√3),(cosa-cosb)^2=0.25
cos(a-b)= cosa cos b+sin a sin b
讲上面两式代入即可得到结果
7已知A B 为钝角且sinA=1/√5,sinB=1/√10,则A+B=
cosA=-2/√5,cosB=-3/√10,cos(A+B)=1/√2