I found: ##tan(theta)=1##

Consider the following diagram:

We can see that the relationships between rectangular and are:

##r=sqrt(x^2+y^2)##

##theta=arctan(y/x)##

and:

##x=rcos(theta)##

##y=rsin(theta)##

Given our expression:

##y=x##

you have:

##cancel(r)sin(theta)=cancel(r)cos(theta)##

and write:

##tan(theta)=1##