求值:cos5π/8cosπ/8=
1个回答

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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