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Sec has a definition where in the value of sec is the reciprocal for the value of cos. Simply sec theta=1/cos theta.

Hope this helps =)
sec theta=2
theta=sec inverse(2)
I don't think you might be able to put it in calculator so change the sec to only cos and reciprocal the value
so its like theta=cos inverse(1/2)
the inverse is the -l on the calculator