If D is any point on the side BC of ΔABC, then:
AB + BC + CA > 2AD
In Δ ABD, AB + BD > AD …(i) (Sum of two sides of a triangle is greater than the third side)
In Δ ACD, AC + DC > AD …(ii) (Sum of two sides of a triangle is greater than the third side)
On adding (i) and (ii)
AB + BD + AC + DC > AD + AD
AB + BC + AC > 2AD