sin8°+sin7°cos15°)÷(cos8°-sin7°sin15°)
=（sin(15-7)+sin7cos15）/（cos(15-7)-sin7sin15）----两角和公式
=(sin15cos7-sin7cos15+sin7cos15)/(cos15cos7+sin7sin15-sin7sin15)
=sin15cos7/cos15cos7
=tan15
=tan(30/2)
=(1-cos30)/sin30
=2-跟3