The Cartesian form of #(r,theta)=(9,(3pi)/4)# is #(x,y)=(-(9sqrt(2))/2,(9sqrt(2))/2)#.
Explanation:
Use the equations #x=rcos(theta)# and #y=rsin(theta)# to get #x=9cos((3pi)/4)=9 * -(sqrt(2))/2=-(9sqrt(2))/2# and #y=9sin((3pi)/4)=9 * (sqrt(2))/2=(9sqrt(2))/2#.