VP=[tex][sin x +sin(\frac{\pi }{3}-x)]^{2}-sinx.sin(\frac{\pi }{3}-x)[/tex]=[tex][2.sin\frac{\pi }{6}.cos(x-\frac{\pi }{6})]^{2}-\frac{1}{2}.[cos(2x-\frac{\pi }{3})+cos\frac{\pi }{3}][/tex]=[tex]cos^{2}(x-\frac{\pi }{6})-\frac{1}{2}.[2.cos^{2}(x-\frac{\pi }{6})-1+cos\frac{\pi }{3}][/tex] [tex]\frac{1}{2}+\frac{1}{4}=\frac{3}{4}[/tex](đpcm)