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cos ^2x - 2cos X -1=0 use foil

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1 Answer

You can solve it by completing the square.

cos^2 x - 2cos x + 1 = 2

(cos x - 1)^2 = 2

cos x = 1 - sqrt(2)

In (0, 2pi), x = 1.998 rad, 2pi-1.998 rad