Soln:
Use formula:
cos2A = 2cos^2(A) - 1
cos2A+1= 2cos^2(A)
(1+cos2A)/2 = cos^2(A)
i.e. cos^2(A) = (1+cos2A)/2
Prove that: cos^2(A+120)+cos^2(A-120)+cos^2(A) = 3/2
Soln:
Use formula:
cos2A = 2cos^2(A) - 1
cos2A+1= 2cos^2(A)
(1+cos2A)/2 = cos^2(A)
i.e. cos^2(A) = (1+cos2A)/2
Prove that: cos^2(A+120)+cos^2(A-120)+cos^2(A) = 3/2
S.E.E. Solution
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Soln: Use formula: cos2A = 2cos^2(A) - 1 cos2A+1= 2cos^2(A) (1+cos2A)/2 = cos^2(A) i.e. cos^2(A) = (1+cos2A)/2
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