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Use this famous equation:sin^2(x) + cos^2(x)=1

sin^2(x) + cos^2(2/5) =1

sin^2(x) =1 - cos^2(2/5) take the square root of both sides.

sin =0.916515...

Sin^2 = 1 - cos^2

          = 1 - 4/25

           = 21/25

 

sin  =  sqrt (21/25) =  sqrt 21 / 5