We have the intresting Law of cosines, so we can use it:
\(c^2=a^2+b^2-2ab\cos \gamma\)
where gamma is angle between a and b.
\(c^2=0,5^2+1,5^2-2*0,5*1,5*\cos \gamma\)
\(c^2=2,5-1,5*\cos \gamma\)
if gamma=90 degrees:
\(c^2=2,5 \Rightarrow c=0,5\)
\(c= \sqrt{2,5-1,5*\cos \gamma}\)
