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求下列各式的值cos7π/12.(2）cos（-61π/12）
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cos7π/12=cos(π/2+π/12)=-sinπ/12=-sin(π/3-π/4)=sinπ/4cosπ/3-sinπ/3cosπ/4
=√2/4(1-√3）=（√2-√6）/4
cos（-61π/12）=cos（61π/12）=cos（5π+π/12)=-cosπ/12=-cos(π/3-π/4)
=-cosπ/3cosπ/4-sinπ/3sinπ/4=-√2/4(1+√3）=-（√2+√6）/4
=√2/4(1-√3）=（√2-√6）/4
cos（-61π/12）=cos（61π/12）=cos（5π+π/12)=-cosπ/12=-cos(π/3-π/4)
=-cosπ/3cosπ/4-sinπ/3sinπ/4=-√2/4(1+√3）=-（√2+√6）/4
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