There is also "solving by assuming the negative". It's a technique for problems like "Given X, prove Y", where you assume "Y is false" and use it to prove "X is

There is also "solving by assuming the negative". It's a technique for problems like "Given X, prove Y", where you assume "Y is false" and use it to prove "X is false", but X is true, therefore Y has to be true. Sometimes it's easier to do it in that direction than X->Y.