函数y=sin(θ-π/2)cos(θ+π/2),θ∈[0,2π/3]的最小值

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函数y=sin(θ-π/2)cos(θ+π/2),θ∈[0,2π/3]的最小值
函数y=sin(θ-π/2)cos(θ+π/2),θ∈[0,2π/3]的最小值

函数y=sin(θ-π/2)cos(θ+π/2),θ∈[0,2π/3]的最小值
y=sin(θ-π/2)cos(θ+π/2)
=-sin(π/2-θ)cos(π/2+θ)
=cosθsinθ
=(sin2θ)/2
θ∈[0,2π/3],2θ∈[0,4π/3],-√2/2≤sin2θ≤1
所以 -√2/4≤y≤1/2
则y最小值为-√2/4

f(x)=sin(θ-pi/2)cos(θ+pi/2)
=sin(θ-pi/2)cos[(θ-pi/2)+pi]
=-sin(θ-pi/2)cos(θ-pi/2)
=-sin(2θ-pi)/2
=sin2θ/2
所以在[0,2pi/3]上的最小值是f(2pi/3)=sin(4pi/3)/2=-根号3/4