Where do I find versA to use in a trig formula?

Versin is an acient Trig function The word is derived from the Latin: sinus versus.

The function is equivalent to

1 − cos(θ) and 2sin²(½θ).

Some here probably though Geno was sipping too much Vino.

You use inverse trig or arc trig (same thing) when you have the ratio and you want to find the angle.

versin(A)  =  1 - cos(A)  

                 =  2sin²(½A)

Can someone (maybe Gino, but he is not here at the moment)  tell me what Gino has done please?

The versine is defined as geno has written it.

 See http://en.wikipedia.org/wiki/Versine 

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Thanks Alan.