f(x)=sin2x-2sin^2(π/2+x)=sin2x-2cos^2x=sin2x-( 1+cos2x)=sin2x-cos2x-1=√2sin(2x-π/4)-1
最小正周期T=2π/2=π 值域为[-√2-1,√2-1]
ABC中，∠BAC=θ,f(θ)=√2sin(2θ-π/4)-1=0
则√2sin(2θ-π/4)=1
,AB=√2,...
已知函数f(x)=sin2x-2sin^2(π/2+x)
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