i don't know abt this theta but i know it does not come in 10th trigonometry
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Hetvi Patel answered this
the versine or versed sine, versin(θ), is a trigonometric function equal to 1 cos(θ) and 2sin 2(½θ). After surfing on net I found it. But if you are a student of class 10th you don't need to go much in details because it will make you confused!!!