see the attached figure below to better understand the problem
Remember that in a unit circle the radius r=1
so
Applying the Pythagorean Theorem in the right triangle
[tex]1^2=(\frac{1}{2})^2+y^2[/tex]
Solve for y
[tex]y^2=1-(\frac{1}{4})[/tex][tex]\begin{gathered} y^2=\frac{3}{4} \\ y=\frac{\sqrt[]{3}}{2} \\ \end{gathered}[/tex]
