Wondrous Arithmetic

Here at home we just had exams again and I helped my daughter preparing for her math exam. We started doing this during the 2020 lock down and we have continued since then. Surprisingly enough I still know most of the math I got in high school more than 30 years ago. This is in stark contrast with other subjects like chemistry which I have mostly forgotten. This time one of the exam topics was trigonometry and during preparation I solved a problem coming to an answer that was different from the one given at the back of the book. It turns out both answers had a different algebraic form but they were numerically identical. To ‘understand’ why they were the same I derived one form out of the other as a proof. I posted the calculation in another post on the blog.