Explanation:
a) BD/AD=AB/AC=AD/CD; BD/AB=AB/BC=AD/AC; AD/AB=AC/BC=CD/AC;
b) if BC=2 and DC=10, then it is possbile to write:
BD/AD=AB/AC=AD/10; BD/AB=AB/2=AD/AC; AD/AB=AC/2=10/AC, then
AC/2=10/AC and AC²=20. According to the Pyphagorean's theorem AD²=AC²+CD² and AD=√120≈10.96