[Asooka]

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.

[Tyr42]

Contrapositive is the fancy word for that technique.

A -> B iff not(B) -> not(A)