No, not necessarily. Cosine theta is equal to 1 only when theta is equal to zero and multiples of 2 pi radians or multiples of 360 degrees. This is because cosine theta is hypotenuse over adjacent, and the ratio 1 only occurs at 0, 360, 720, etc. or 0, 2 pi, 4 pi, etc.
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Let 'theta' = A [as 'A' is easier to type] sec A - 1/(sec A) = 1/(cos A) - cos A = (1 - cos^2 A)/(cos A) = (sin^2 A)/(cos A) = (tan A)*(sin A) Then you can swap back the 'A' with theta
cos2(theta) = 1 cos2(theta) + sin2(theta) = 1 so sin2(theta) = 0 cos(2*theta) = cos2(theta) - sin2(theta) = 1 - 0 = 1
It's 1/2 of sin(2 theta) .
Remember that tan = sin/cos. So your expression is sin/cos times cos. That's sin(theta).
cosine (90- theta) = sine (theta)