cos9π/4+tan（-7π/6）+sin21π?、
问题描述：
cos9π/4+tan（-7π/6）+sin21π?、
答
原式=cos(2π+π/4)-tan(π+π/6)+sin(20π+π)
=cosπ/6-tanπ/6+sinπ
=√3/2-√3/3+0
=√3/6