[tex]\bf x=rcos(\theta)\qquad \qquad y=rsin(\theta)\\\\
-----------------------------\\\\
25xy=225\implies xy=\cfrac{225}{25}\implies xy=9
\\\\\\
rcos(\theta)\cdot rsin(\theta)=9\impliedby \textit{common factor}
\\\\\\
r[cos(\theta)sin(\theta)]=9\implies r=\cfrac{9}{cos(\theta)sin(\theta)}[/tex]
