Absur Khan Siam wrote:Let's extend AC and define two points $F$ and $E$ such that $CE = BC$ and
$CD = CF$.
By some angle chasing,
$\angle DEF = \angle EDF \Rightarrow DF = EF$
$\angle DFA = \angle FAD \Rightarrow DF = AD$
$\therefore AD = EF$
Now ,
$AD + CD = EF + CF = CE = BC = 2017$
ahmedittihad wrote:What is cosine law?
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