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Gradient checking
Approximating derivatives:
(Large triangle (+/- triangle))
$\frac{f(\Theta + \epsilon) - f(\Theta - \epsilon)}{2 \epsilon} ~ g(\Theta)$
$f'(\Theta) = \lim_{\epsilon->0} \frac{f(\Theta + \epsilon) - f(\Theta - \epsilon)}{2 \epsilon}$
Approx error is in $O(\epsilon^2)$