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问题: 急要!!!!!帮帮忙....

1.cos20cos40cos60cos80=? (数字指度数)
2.已知cos2θ=(根号2)/3,则(sinθ)^4+(cosθ)^4的值为?
3.已知sinx-cosx=(根号2)/2,则cos4x=?

解答:

1.cos20cos40cos60cos80=? (数字指度数)
=16sin20cos20cos40cos60cos80/16sin20
=8sin40cos40cos60cos80/16sin20
=4sin80cos80cos60/16sin20
=2sin160cos60/16sin20
=sin20/16sin20=1/16
2.已知cos2θ=(根号2)/3,则(sinθ)^4+(cosθ)^4的值为?
cos2θ=(根号2)/3
(sin2θ)^=1-(cos2θ)^=7/9
(sinθ)^4+(cosθ)^4=[(sinθ)^+(cosθ)^]^-2(sinθcosθ)^
=1-[(sin2θ)^/2] =1-(7/18)=11/18
3.已知sinx-cosx=(根号2)/2,则cos4x=?
(sinx)^-2sinxcosx+(cosx)^=1/2
sin2x=1/4 (sin2x)^=1/16
(cos2x)^=15/16
cos4x=(cos2x)^-(sin2x)^
=(15/16)-(1/16)=7/8