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7sin^2x + cos^2x = 4
Split the 7 on the left hand side as 6 and 1
6sin^2x + sin^2x + cos^2x = 4
Since sin^2x + cos^2x = 1
6sin^2x + 1 = 4
6sin^2x = 3
sin^2x = 1/2
sin x = 1/sqrt(2) or -1/sqrt(2)
Therefore
cos x = 1/sqrt(2) or -1/sqrt(2) (cos x = sqrt (1 - sin^2x)
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