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![[Post New]](/templates/default/images/icon_minipost_new.gif) 7 Apr 2008 18:49:46 IST
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try this fundamental method;
cos3A=4cos^3A-3cosA
cos^3A=1/4(cos3A+3cosA)
putting in eq.
kcos3A= 1/4(cos3A+3cosA)+1/4[cos(360+3A)+3cos(120+A)]+1/4[cos(360-3A)+3cos(120-A)]
=1/4[cos3A+3cosA]+1/4[cos3A+3cos(120+A)]+1/4[cos3A+cos(120-A)]
=3/4cos3A+3/4[cosA+cos(120+A)+cos(120-A)]
=3/4cos3A+3/4[cosA+2cos120cosA]
=3/4cos3A+3/4[cosA-cosA]
=3/4cos3A
therefore=3/4
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