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Do you mean if sin theta = 2/5, then what is the exact value of cos 2 theta? If so, then you can apply one of the double angle formulae, ie 

cos 2 theta = 1-(sin theta)^2

                  = 1 - (2/5)^2


What do you mean by sin data? Do you mean sin20 = 2/5?

if sin data =2/5 what is the exact value of cos 20?

if sin data =2/5 what is the exact value of cos 20?

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