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[tex]

V_{freestream}sin \beta_i - \sum_{j=1}^n \displaystyle\frac{\lambda_i}{2\pi} \int \displaystyle\frac{d\theta_{ij}}{dn_i} ds_j = 0[/tex]

where n is the panel number, i is the control point at which the vortex strength is being calculated and j is the panel which is inducing some vortex at i, [tex]\lambda_i[/tex] is the vortex strength at i and

[tex] \theta_{ij} = \arctan{\displaystyle\frac{y_i-y_j}{x_i-x_j}}[/tex]

My question is what is the value of [tex]\theta_{ij}[/tex] when i = j?