The Fitzhugh-Nagumo model for the electrical impulse in a neuron states that, in the absence of relaxation effects, the electrical potential in a neuron v(t) obeys the differential equation

where a is a positive constant such that 0 < a < 1.
(a) For what values of v is v unchanging (that is, dv/dt = 0)?
(b) For what values of v is v increasing?
(c) For what values of v is v decreasing?

dv -v[v? (1 + a)v + a] dt