question_answer

Let \[f(x)=cos\,\,\times \,\,sin2x\], then

A)  \[\min \,\,f\,\,(x)=-\frac{1}{3\sqrt{3}}\,\,for\,\,x\,\,\in [-\pi ,\pi )\]

B)  \[\min \,\,f(x)>-\frac{9}{7}\,\,or\,\,-\frac{7}{9}\,\,for\,\,x\,\,\in [-\pi ,\pi ]\]

C)  \[\min \,\,f\,\,(x)>-\frac{1}{9}\,\,for\,\,x\,\,\in [-\pi ,\pi ]\]     

D)  \[\min \,\,f\,\,(x)>-\frac{2}{9}\,\,for\,\,x\,\,\in [-\pi ,\pi ]\]