A mathematical model for the rate at which a drug disseminates into the bloodstream is given by dx dt = r − kx, where r and k are positive constants. The function x(t) describes the concentration of the drug in the bloodstream at time t. (a) Since the DE is autonomous, use the phase portrait concept of Section 2.1 to find the limiting value of x(t) as t → [infinity]. lim t→[infinity] x(t) = r k​