I learned this in the US, in high school geometry (Sophomore year). It was also called indirect proofs by textbook, and it’s when I first fell in love with mat

I learned this in the US, in high school geometry (Sophomore year). It was also called indirect proofs by textbook, and it’s when I first fell in love with math. Note: Linear algebra killed said love of math for me ever so slightly, when the instructor chose a terrible textbook.