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We study ${\cal N}=2 SO(2N+1)$ SYM theory in the context of matrix model. By adding a superpotential of the scalar multiplet, $W(\Phi)$, of degree 2N+2, we reduce the theory to ${\cal N}=1$. The 2N+1 distinct critical points of $W(\Phi)$ allow us to choose a vacuum in such a way to break the gauge group to its maximal abelian subgroup. We compute the free energy of the corresponding matrix model in the planar limit and up to two vertices. This result is then used to work out the effective superpotential of ${\cal N}=1$ theory up to one-instanton correction. At the final step, by scaling the superpotential to zero, the effective U(1) couplings and the prepotential of the ${\cal N}=2$ theory are calculated which agree with the previous results.
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