求值:cos5π/8cosπ/8=

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求值:cos5π/8cosπ/8=求值:cos5π/8cosπ/8=求值:cos5π/8cosπ/8=cos(5π/8+π/8)=cos5π/8cosπ/8-sin5π/8sinπ/8=-√2/2co

求值:cos5π/8cosπ/8=
求值:cos5π/8cosπ/8=

求值:cos5π/8cosπ/8=
cos(5π/8+π/8)=cos5π/8cosπ/8-sin5π/8sinπ/8=-√2/2
cos(5π/8-π/8)=cos5π/8cosπ/8+sin5π/8sinπ/8=0
两式相加2cos5π/8cosπ/8=-√2/2
cos5π/8cosπ/8=-√2/4
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