（cos7°-cos8°cos15°）/（cos23°-cos8°cos15°）=

（cos7°-cos8°cos15°）/（cos23°-cos8°cos15°）=
（cos7°-cos8°cos15°）/（cos23°-cos8°cos15°）=

（cos7°-cos8°cos15°）/（cos23°-cos8°cos15°）=
cos7°=cos(15°-8°)=cos8°cos15°+sin8°sin15°,那么cos7°-cos8°cos15°=sin8°sin15°；
cos23°=cos(8°+15°)=cos8°cos15°-sin8°sin15°,那么cos23°-cos8°cos15°=-sin8°sin15°
所以原式=(sin8°sin15°)/(-sin8°sin15°)=-1