Question: convert variable

hi

 how can I convert cartesian variable to  polar variable in ordinary differential equations by using maple.

for example,

x dot= x+y   and      y dot= -x+y  convert to   r dot =r(1-r^2)          &       theta dot=-1

by using,       x=r*sin(theta)       &       y=r* cos(theta)      

           &       r^2=x^2=y^2 & tan(theta)=y/x .

 

 

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