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An adaptive output-feedback control scheme is proposed for a class of nonlinear systems with unknown constant parameters, including the high-frequency gain, and unmodeled dynamics. With priori knowledge of the bounds of the parametric uncertainties, the smooth projection method and the discontinuous projection method are incorporated into the adaptive laws to prevent parameter drifts, respectively. The control schemes do not require a priori knowledge of high-frequency gain sign. It is proved that under the proposed control laws, all the closed loop signals are bounded and the output asymptotically converges to zero.