cos12°cos24°cos48°cos84°
觀察知：(最大的度數先不用管）
48為24的2倍
24為12的2倍
且均為餘弦
針對此類題
應乘以一個最小度數的正弦
如下：
=sin12°cos12°cos24°cos48°cos84°/sin12°
=2sin12°cos12°cos24°cos48°cos84°/2sin12°
=sin24°cos24°cos48°cos84°/2sin12°
=sin48°cos48°cos84°/4sin12°
=sin96°cos84°/4sin12°
=2sin(180°-96°)vos84°/8sin12°
=2sin84°cos84°/8sin12°
=sin168°/8sin12°
=sin(180°-168°)/8sin12°
=sin12°/8sin12°
=1/8
cos12°cos24°cos48°cos84°
觀察知：(最大的度數先不用管）
48為24的2倍
24為12的2倍
且均為餘弦
針對此類題
應乘以一個最小度數的正弦
如下：
=sin12°cos12°cos24°cos48°cos84°/sin12°
=2sin12°cos12°cos24°cos48°cos84°/2sin12°
=sin24°cos24°cos48°cos84°/2sin12°
=sin48°cos48°cos84°/4sin12°
=sin96°cos84°/4sin12°
=2sin(180°-96°)vos84°/8sin12°
=2sin84°cos84°/8sin12°
=sin168°/8sin12°
=sin(180°-168°)/8sin12°
=sin12°/8sin12°
=1/8