sinx=根号5-1/2,求sin2(x-π/4)的值 sin2(x-π/4)=sin(2x-π/2)=-sin(π/2-2x)=-cos2x=-(1-2sin²x)=2sin²x-1=2[(√5-1)/2]²-1=2*(6-2√5)/4-1=2*2(3-√5)/4-1=3-√5-1=2-√5为什么不能=sin(2x-π/2)=cos(2x-π/2+π/2)=cos
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![sinx=根号5-1/2,求sin2(x-π/4)的值 sin2(x-π/4)=sin(2x-π/2)=-sin(π/2-2x)=-cos2x=-(1-2sin²x)=2sin²x-1=2[(√5-1)/2]²-1=2*(6-2√5)/4-1=2*2(3-√5)/4-1=3-√5-1=2-√5为什么不能=sin(2x-π/2)=cos(2x-π/2+π/2)=cos](/uploads/image/z/6869970-18-0.jpg?t=sinx%3D%E6%A0%B9%E5%8F%B75-1%2F2%2C%E6%B1%82sin2%28x-%CF%80%2F4%29%E7%9A%84%E5%80%BC+sin2%EF%BC%88x-%CF%80%2F4%EF%BC%89%3Dsin%EF%BC%882x-%CF%80%2F2%EF%BC%89%3D-sin%28%CF%80%2F2-2x%29%3D-cos2x%3D-%281-2sin%26%23178%3Bx%29%3D2sin%26%23178%3Bx-1%3D2%5B%28%E2%88%9A5-1%29%2F2%5D%26%23178%3B-1%3D2%2A%286-2%E2%88%9A5%29%2F4-1%3D2%2A2%283-%E2%88%9A5%29%2F4-1%3D3-%E2%88%9A5-1%3D2-%E2%88%9A5%E4%B8%BA%E4%BB%80%E4%B9%88%E4%B8%8D%E8%83%BD%3Dsin%EF%BC%882x-%CF%80%2F2%EF%BC%89%3Dcos%282x-%CF%80%2F2%2B%CF%80%2F2%EF%BC%89%3Dcos)
sinx=根号5-1/2,求sin2(x-π/4)的值 sin2(x-π/4)=sin(2x-π/2)=-sin(π/2-2x)=-cos2x=-(1-2sin²x)=2sin²x-1=2[(√5-1)/2]²-1=2*(6-2√5)/4-1=2*2(3-√5)/4-1=3-√5-1=2-√5为什么不能=sin(2x-π/2)=cos(2x-π/2+π/2)=cos
sinx=根号5-1/2,求sin2(x-π/4)的值
sin2(x-π/4)
=sin(2x-π/2)
=-sin(π/2-2x)
=-cos2x
=-(1-2sin²x)
=2sin²x-1
=2[(√5-1)/2]²-1
=2*(6-2√5)/4-1
=2*2(3-√5)/4-1
=3-√5-1
=2-√5
为什么不能
=sin(2x-π/2)
=cos(2x-π/2+π/2)
=cos2x
sinx=根号5-1/2,求sin2(x-π/4)的值 sin2(x-π/4)=sin(2x-π/2)=-sin(π/2-2x)=-cos2x=-(1-2sin²x)=2sin²x-1=2[(√5-1)/2]²-1=2*(6-2√5)/4-1=2*2(3-√5)/4-1=3-√5-1=2-√5为什么不能=sin(2x-π/2)=cos(2x-π/2+π/2)=cos
∵sina≠cos(a+π/2)
∴sin(2x-π/2)≠ cos(2x-π/2+π/2)
cos(a+π/2)= - sina
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