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The inverse (negatives) of sine, cosine, and tangent are used to calculate the angle theta (or whatever you choose to name it).
Initially it is taught that opposite over hypotenuse is equal to the sine of theta
sin(theta) = opposite/hypotenuse
So it can be said that
theta = sin-1 (opp/hyp)
This works the same way with cosine and tangent
In short the inverse is simply what you use when you move the sin, cos, or tan to the other side of the equation generally to find the angle
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