2sin²12分之7π-1=

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2sin²12分之7π-1=
2sin²12分之7π-1=

2sin²12分之7π-1=

2sin²7π/12-1
=-(1-2sin²7π/12)
=-cos7π/6
=-cos(π+π/6)
=cosπ/6=根号3/2