if cos is 2/5 what is sin on a unit circle

Using

1 - cos²x = sin²x   .... we have...

1 - (2/5)² = sin²(x)

1 - (4/25) = sin²(x)

21/25 = sin²(x)      take the square root of both sides

±√(21 / 25)  = [±(1/5)√21 ] = sin(x)