If a and to are mitt nectom and or is ( 2^{text {le }} ) b/w them there find the value of ( frac{sin theta}{2} )

( vec{a} cdot vec{b}=|vec{a}||vec{b}| cos theta=cos theta )
( sin theta=sqrt{1-cos ^{2} theta}=sqrt{1-(vec{a} cdot vec{b})^{2}} )
( frac{sin theta}{2}=frac{sqrt{1-(vec{a} cdot vec{b})^{2}}}{2} )