We investigate mixed nonlinear integro-differential equations (MNIDEs) in general, utilizing the concept of rationalized Haar (RH) wavelet. The complexity of the MNIDE solution is known to everyone. For this purpose, we present a numerical method by applying the RH wavelet to approximate solutions of the MNIDE of the second kind in the complex plane. At first, we describe a continuous integral operator . Also, under mild assumptions, the Banach fixed point theorem ensures that the integral operator has a unique solution. Moreover, we give a result for error and compute the rate of convergence. Employing an algorithm, we present some illustrative examples to demonstrate the performance of this approach.

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