Prove that : $MB=MA+MC$

Question

Problem :

Let $ABC$ be a equilateral triangle

$C$ : circumscribed circle of $ABC$

$M$ point in arc $AB$ , $M≠B$

$D$ point in $[BM]$ such : $AD=AM$

Prove that :

$MB=MA+MC$

Can you give me some hints or idea to approach it ?