Let \(\alpha\) and \(\beta\) be the roots of the equation \(x^{2} + qx\) \(- r = 0\), where \(p \neq 0\). If \(p,q\) and \(r\) be the consecutive terms of a non constant G.P. and \(\frac{1}{\alpha} + \frac{1}{\beta} = \frac{3}{4}\), then the value of \((\alpha - \beta)^{2}\) is