`cosA+cos(12 0^(@)+A)+cos(12 0^(@)-A)=`

1.	`cosA+cos(12 0^(@)+A)+cos(12 0^(@)-A)=`
Answer» LHS`=cosA+cos(120^(@)+A)+cos(120^(@)-A)` `=cosA+(cos120^(@)cosA-sin120^(@)sinA) + cos120^(@)cosA+sin120^(@)sinA` `=cosA+2cos120^(@)cosA` `=cosA+2cos(180^(@)-60^(@))cosA` `cosA-2(1/2)cosA` `=cosA-cosA=0=` RHS Hence proved

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