Question 595489
sec(x) = 4


1/cos(x) = 4


1 = 4*cos(x)


1/4 = cos(x) 


cos(x) = 1/4


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cos(2x)


2*(cos(x))^2 - 1


2*(1/4)^2 - 1


2*(1/16) - 1


1/8 - 1


-7/8



So if sec(x) = 4, then cos(2x) = -7/8