Consider the second order system x x(x+x) x = x x2(x + x). x2 x = x2-x = a) Write differential equations for the variables r and in polar coordinates (x x2 = rsin) in a way that their derivatives may depend on r and 0, but not on x1, x2. b) Draw phase portraits (in x1, x2 coordinates) for the cases 0. rcos 0,