cot15°+1/cot15°-1=?
问题描述：
cot15°+1/cot15°-1=?
答
(cot15°+1)/(cot15°-1)
=(cos15°/sin15+1)/(cos15°/sin15-1)
=(cos15+sin15)/(cos15-sin15)
=(cos15+sin15)(cos15+sin15)/[(cos15-sin15)(cos15+sin15)]
=(cos^2 15+sin^2 15+2cos15sin15)/(cos^2 15-sin^2 15)
=(1+sin30)/cos30
=(1+1/2)/(√3/2)
=√32cos15sin15 why----->sin30(cos^2 15-sin^2 15) why--->cos30这个是公式啊sin2a=2sinacosacos2a=cos^2 a -sin^2 a