I recently came across this splendid resource for introducing Sine and Cosine rule to students.

Proving the Sine and Cosine rule

The proofs for these rules are relatively simple, but getting a class of teenagers to engage with it is a different matter! These worksheets give you the proofs, step by step, but all jumbled up. Students must rearrange the stages in order to create a proof. It worked brilliantly!

Thank you @mrslack_maths