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LHS
`2cospi/13 cos(9pi)/(13)+cos(3pi)/(13)+cos(5pi)/(13)=0`
`=2cospi/13 cos(9pi)/(13) + 2cos((5pi)/(13)+(3pi)/(13))/(2) cos((5pi)/(13)-(3pi)/(13))/(2)`
`=2cospi/13cos(9pi)/(13)+2cos(4pi)/(13)cospi/13`
`=2cospi/13(cos(9pi)/(13) + cos(4pi)/(13))`
`=2cospi/13[2cos((9pi)/(13)+(4pi)/(13))/(2) cos((9pi)/(13)-(4pi)/(13))/(2)]`
`=2cospi/13[(2cospi/2cos(5pi)/(26))]`
`=4cospi/13 cos(5pi)/(26) xx cospi/2`
`=4cospi/13 cos(5pi)/(26) xx 0` `(therefore cospi/2=0)`
=0 = RHS Hence Proved.
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