a,b为锐角,cos(a-b/2)=根号3/2,sin(a/2-b)=-1/2,求cos(a+b)
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a,b为锐角,cos(a-b/2)=根号3/2,sin(a/2-b)=-1/2,求cos(a+b)
sin(a/2-b)=-1/2,a,b为锐角.
cos(a/2-b)=√[1-sin^2(a/2-b)]=√3/2,
sin[2(a/2-b)]=2*sin(a/2-b)*cos(a/2-b)=2*(-1/2)*√3/2=-√3/2.
即,sin(a-2b)=-√3/2.
cos(a-2b)=√[1-sin^2(a-2b)]=1/2.
cos(a-b/2)=根号3/2,
cos[2(a-b/2)]=2cos^2(a-b/2)-1=2*3/4-1=1.
即,cos(2a-b)=1,
sin(2a-b)=√[1-cos^2(2a-b)]=0.
cos(a+b)=cos[(2a-b)-(a-2b)]
=cos(2a-b)*cos(a-2b)+sin(2a-b)*sin(a-2b)
=1*1/2+0*(-√3/2)
=1/2.
即,cos(a+b)=1/2.
详细吧.
cos(a/2-b)=√[1-sin^2(a/2-b)]=√3/2,
sin[2(a/2-b)]=2*sin(a/2-b)*cos(a/2-b)=2*(-1/2)*√3/2=-√3/2.
即,sin(a-2b)=-√3/2.
cos(a-2b)=√[1-sin^2(a-2b)]=1/2.
cos(a-b/2)=根号3/2,
cos[2(a-b/2)]=2cos^2(a-b/2)-1=2*3/4-1=1.
即,cos(2a-b)=1,
sin(2a-b)=√[1-cos^2(2a-b)]=0.
cos(a+b)=cos[(2a-b)-(a-2b)]
=cos(2a-b)*cos(a-2b)+sin(2a-b)*sin(a-2b)
=1*1/2+0*(-√3/2)
=1/2.
即,cos(a+b)=1/2.
详细吧.
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