sin8°+sin7°cos15°
=sin(15°-7°) +sin7°cos15°
=sin15°cos7°-sin7°cos15°+sin7°cos15°
=sin15°cos7°
cos8°-sin7°sin15°
=cos(15°-7°) -sin7°sin15°
=cos15°cos7°+sin7°sin15°-sin7°sin15°
=cos15°cos7°
故
（sin8°+sin7°cos15°）/（cos8°-sin7°sin15°）
=sin15°cos7°/cos15°cos7°
=tan15°