In a triangle, When one angle is 30 degrees(given).
Then the other two angles are 90 and 60 degrees.
Express in the form of ratio:
30:60:90, the sides are in the ratio of 1: \sqrt{3} :2.
The sine function is defined as the ratio of opposite side to the hypotenuse.
For 30 degrees, the ratio is 1/2.
Therefore, \sin30^0=\frac{1}{2}