a·b=cos3x/2cosx/2-sin3x/2sinx/2=cos(3x/2+x/2)=cos2x
|a+b| =√[(cos3x/2+cosx/2)^2+(sin3x/2-sinx/2)^2]
=√[(cos3x/2)^2+(sin3x/2)^2+(cosx/2)^2+(sinx/2)^2+2cos3x/2cosx/2-2sin3x/2sinx/2]
=√(2+2(cos3x/2cosx/2-2sin3x/2sinx/2))
=√(2+2cos2x)
=√(2+2(cosx)^2-2(sinx)^2)
=√4(cosx)^2
=2cosx 因为x属于[0,π、2],所以cosx大于等于0,开根号不用加负号