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