First principles

In my A-level class I was trying to solve a question but could not for the life of me get the correct result. I spent about 20mins on it, googled the question and asked the other A-level teacher for help. Even with all that help we could not solve it. Two students stayed behind and missed their break to put our brains together to find the solution. Much to my delight my girl managed to find the solution! It’s so elegant. I was super proud of her!