$\cos 20^{\circ} + \cos 30^{\circ} + \cos 40^{\circ} = $

  • A
    $1 - 2 \sin 10^{\circ} \sin 15^{\circ} \sin 20^{\circ}$
  • B
    $4 \cos 20^{\circ} \cos 30^{\circ} \cos 40^{\circ}$
  • C
    $4 \cos 10^{\circ} \cos 15^{\circ} \cos 20^{\circ}$
  • D
    $4 \cos 25^{\circ} \cos 30^{\circ} \cos 35^{\circ}$

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