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"How to solve it : a new aspect of mathematical method" is a book regarding problem-solving skill in maths and it certainly benefits me. Not only does it conclude the Descartes' way of solving problems, but it gives concrete example too.
Frankly, a lot of techniques stated have already been mentioned by our math teachers as well. For example, techniwue of induction and deduction have been widely used in solving algebric questions, planning in advance is one of the most useful techniques in geometry and realising symmetry is absolutely helpful in learning about different shapes. It is like a summary of a bunch of math skills ?with some extension by the writer.
In case you are have trouble doing sum, you may want to have a look on it.