We are given: cos theta = 1/3. Thus, adj side = 1 and hyp = 3. Using the Pythagorean Theorem to find the length of the opposite side, represented by x.
1^2 + opp^2 = 3^2, or 1 + x^2 = 9, or x^2 = 8. Here, x could be either sqrt(8) or -sqrt(8): sqrt(8) if angle theta is in QI, and -sqrt(8) if in QIV.
Thus, the sine of angle theta could be either sqrt(8)/3 or -sqrt(8)/3.