Đề bài

Chứng minh rằng:

a) ${\rm{sin}}3x = 4{\rm{sin}}x{\rm{sin}}\left( {{{60}^ \circ } - x} \right){\rm{sin}}\left( {{{60}^ \circ } + x} \right)$;

b) $\frac{{{\rm{sin}}x - {\rm{sin}}2x + {\rm{sin}}3x}}{{{\rm{cos}}x - {\rm{cos}}2x + {\rm{cos}}3x}} = {\rm{tan}}2x$.