Answer:
The positive displacement from the midpoint of its motion at the speed equal one half of its maximum speed is 3.56 cm.
Explanation:
Maximum speed is  :
             v (max) = Aω
Speed v at any displacement y is given by Â
[tex]v^{2}[/tex] = [tex]w^{2}[/tex] ([tex]A^{2}[/tex] - [tex]y^{2}[/tex]) Â ........................................................ Â i
And,
        v = [tex]\frac{1}{2}[/tex] v (max) Â
     or,  2 × v = Aω   ....................................................  ii
Eliminating  ω from equations i and ii,
            [tex]\frac{1}{4}[/tex] [tex]A^{2}[/tex]  [tex]w^{2}[/tex]  =  [tex]w^{2}[/tex]  ( [tex]A^{2}[/tex]  - [tex]y^{2}[/tex])
           or, [tex]y^{2}[/tex] =  ([tex]\frac{3}{4}[/tex]) [tex]A^{2}[/tex]  =([tex]\frac{3}{4}[/tex]) [tex]2.18^{2}[/tex]
          or,  y =  3.56 cm.