2025 SJI PRELIMS P1 Q3

\(ABD\) is a triangle, \(ADC\) is a straight line, \(AB=8\text{ cm}\), \(AD=9\text{ cm}\), and \(BD=10\text{ cm}\). Without using a calculator, find the exact value of \(\cos\angle BDC\).

Find the exact value of \(\cos\angle BDC\).[2]