This can be proved by phythagores theorem.
First let us draw a perpendicular BD from B to AC.
we get three right angled triangles : ABC, BCD, and ADB
AB² = AC·AD , BC² = AC·CD
AB² + BC² = AC·AD + AC·CD
AB² + BC² = AC ( AD + CD )
AB² + BC² = AC(AC)
AB² + BC² = AC²