因为
π/4-θ=π/2-(π/4+θ)
所以sin(π/4-θ)=sin[π/2-(π/4+θ)]=cos(π/4+θ)
所以
sin(π/4+θ)*sin(π/4-θ)
=sin(π/4+θ)*cos(π/4+θ)
=1/2sin(π+2θ)
=-1/2sin2θ=2/5
所以sin2θ=-4/5
cos2θ=3/5或者-3/5
sin(π/4+θ)*sin(π/4-θ)=2/5,则cos2θ=
sin(π/4+θ)*sin(π/4-θ)=2/5,则cos2θ=
数学人气:357 ℃时间:2020-05-01 22:30:16
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