Solve \(tan^{-1} (\frac xy)-tan^{-1}(\frac {x-y}{x+y})\) is equal to
If the coefficient of x7 in expansion of \((ax- \frac{1}{bx^2})^{13}\) is equal to the coefficient of x-5 in expansion of \((ax + \frac{1}{bx^2})13\), then a4b4 is ______