Triangular spider web

In this rectangular window frame, a spider begins to weave its web. The basic framework consists of three threads, which start from points \(A, B,\) and \(C\) and are glued together at point \(D.\)

What is the length \(l_A \) of the first thread in centimeters?

Assumptions: Without tension, all threads have the same length, but they are stretched many times their original length. Thus, the tension force \(F_i \approx - k l_i \) acting along a thread is approximately proportional to its total length \(l_i \). While the lengths \(l_i \) of the threads can be different, the spring constant \(k \) is the same for all threads. Apart from the tension, no further forces act on the spider threads.