" 13."(cos^(3)10^(@)+cos^(3)110^(@)+cos^(3)130^(@))=

cos^(3)110^(@)+cos^(3)10^(@)+cos^(3)130^(@)=

cos ^(3) 110 ^(@) + cos ^(3) 10^(@) + cos ^(3) 130^(@)=

cos ^(3) 110 ^(@) + cos ^(3) 10^(@) + cos ^(3) 130^(@)=

8 cos^(3) 10^(@) -6 cos 10^(@) =

(cos^(3)21^(@)+cos^(3)39^(@))/(cos21^(@)+cos39^(@))=

cos130^(@)+cos110^(@)+cos10^(@)=0

cos130^(@)+cos110^(@)+cos10^(@)=0

i) Prove that: cos^(3)2A+3cos2A = 4(cos^(6)A-sin^(6)A) ii) Prove that: 4(cos^(3)10^(@) + sin^(3)20^(@)) = 3(cos10^(@)+sin20^(@))

Prove the following 4(cos^(3)10^(@)+sin^(3)20^(@))=3(cos10^(@)+sin20^(@))

Prove that, 4 (cos^(3) 10^(@) + sin^(3) 20 ^(@))=3 (cos 10^(@) + sin 20^(@))