In the adjoining figure, if AD, AE and BC are tangents to the circle at D, E and F respectively. Then,
2AD=AB+BC+CA
We know that
AD=AE [Tangents from an external point to the circle are equal in length]
⇒AD=AB+BE
AD=AB+BF [since, BE=BF] ……(1)
Also,
AD=AC+CD
⇒AD=AC+CF [since, CD=CF] ……(2)
Adding equations (1) and (2)
AD+AD=AB+BF+CF+AC
⇒2AD=AB+AC+BC