(sin20*sin10)/(cos20*cos10)
问题描述：
(sin20*sin10)/(cos20*cos10)
答
sin20=2sin10cos10
cos20=1-2(sin10)^2
原式=2（sin10）^2/[1-2(sin10)^2]
=-1+1/[1-2(sin10)^2]
=-1+1/cos20
=sec20-1