cos兀/7+cos2兀/7+……cos6兀/7

cos兀/7+cos2兀/7+……cos6兀/7
cos兀/7+cos2兀/7+……cos6兀/7

cos兀/7+cos2兀/7+……cos6兀/7
根据cos(π-α)=-cosα
∴cos6π/7=-cosπ/7
cos5π/7=-cos2π/7
cos4π/7=-cos3π/7
∴cos6π/7+cosπ/7=0
cos5π/7+cos2π/7=0
cos4π/7+cos3π/7=0
∴cos兀/7+cos2兀/7+……cos6兀/7=0