tan9*tan27+tan27*tan54+tan54*tan9
问题描述：
tan9*tan27+tan27*tan54+tan54*tan9
都是说度数,本来是π/20,3π/20+.
急啊!
答
tan9*tan27+tan27*tan54+tan54*tan9
=tan9(tan27+tan54)+tan27*tan54
=tan9tan81(1-tan27tan54)+tan27tan54
=1-tan2tan54+tan27tan54
=1