Let \({a_k}\) and \({b_k}\), \(πβN\), be two G.P.s with common ratios \(r_1\) and \(r_2\) respectively such that \(a_1= b_1=4\) and \(r_1<r_2\). Let \(c_k=a_k+b_k\), \(kβN\). If \(c_2=5\) and \(c_3=\frac {13}{4}\), then \( \displaystyle\sum_{k=1}^{ββ}c_kβ(12a_6+8b_4)\) is equal to _____ .