Event-triggered control systems are systems in which the control signal is recomputed when the plant's output signal leaves a triggering-set. There has been recent interest in event-triggered control systems as a means of reducing the communication load in control systems. This paper re-examines a problem [1] whose solution characterizes triggering-sets that minimize a quadratic control cost over a finite horizon subject to a hard constraint on the number of times the feedback control is computed. Computational complexity confined prior solutions of this problem to scalar linear systems. This paper presents an approximate solution that is suitable for multidimensional linear systems. This approximate solution uses families of quadratic forms to bound the value functions generated in solving the probelm. This approach has a computational complexity that is polynomial in state-space dimension and horizon length. This paper's results may therefore provide a basis for developing ...