Question

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cos10cos20sin50

Answer 1

 $cos10*cos20*sin 50 = cos10*cos20*cos40 \\ \frac{2sin10*cos10*cos20*cos40}{2sin10 } = \frac{sin20*cos20*cos40}{2sin10 } = \\ \frac{sin40*cos40}{4*sin10} = \frac{sin80}{8*sin10} = \frac{1}{8}*ctg10$