First, show that (0,0) is the only equilibrium of the differential equation (multiply x by x and y by y) d ■x(t) = 4x + 2y − x(x² + y²) d - dt³ (t) = −2x + y − y(x² + y²) Next, write the differential equation in polar co-ordinates [recall that x(t) = rcos(0).] Then, prove that there is at least one limit cycle