Looking for mathematical problem-solving skills, such as method of substitution, undetermined coefficient method, reduction to absurdity, scaling method, etc ... What else can you use?

Matching method, factorization method, method of substitution, discriminant method and Vieta theorem, undetermined coefficient method, undetermined coefficient method, reduction to absurdity method, area method, geometric transformation method, direct deduction method, verification method, special element method, exclusion method, screening method, graphic method, analytic fill-and-dig method (geometry). I really can't think of anything else. It's good to master these.