向量a•向量b
=|a|*|b|*cos96°
=2cos12°*4cos24°cos48°cos96°
=8cos12°cos24°cos48°cos96°
=8sin12°cos12°cos24°cos48°cos96°/sin12°
=4sin24°cos24°cos48°cos96°/sin12°
=2sin48°cos48°cos96°/sin12°
=sin96°cos96°/sin12°
=(1/2)sin192°/sin12°
=-(1/2)sin12°/sin12°
=-1/2