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**Mathstud28** I was reading a book for reviewing the GRE for mathematics...and I came across this problem "Describe the family of curves described by $\displaystyle x\cdot{dy}=y\cdot{dx}$"...so I preceeded to go $\displaystyle \frac{dy}{y}=\frac{dx}{x}$...which means that $\displaystyle ln|y|=ln|x|+C$ which means that $\displaystyle y=Cx$...so I picked choice A" The set of lines intersecting the origin"...but that was incorrect..the asnwer was the set of all circles with center (0,0)...could someone explain?