Use the Half-Angle Formula for Sine which follows directly from the Double Angle Formula for Cosine which follows directly from the Sum Formula for Cosine. See Double Angle and Half Angle Formulas for a complete discussion.
And
From the unit circle:
Since the half-angle formula for sine is:
where you select the sign based on the quadrant in which exists.
We can write, since is in the first quadrant:
John
My calculator said it, I believe it, that settles it
