Secondary school mathematics
Practice questions

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Madasmaths (available at https://madasmaths.com)  a veritable goldmine of mathematics questions.

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Underground Mathematics (https://undergroundmathematics.org) who have a lot of good practice problems for A Level mathematics.

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Isaac Physics (https://isaacphysics.org), for both the practice questions as well as their notes (https://isaacphysics.org/concepts?stage=all).
General resources

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The textbook "Further Pure Mathematics" (and the other books in the series) by Linda Bostock and Sue Chandler. Available to borrow (for free) from the Internet Archive (https://archive.org/details/furtherpuremathe00bost).

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Math Centre https://mathcentre.ac.uk (in particular their "teach yourself" section, available at https://mathcentre.ac.uk/types/#h4)

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HELM (Helping Engineers Learn Mathematics): features notes and practice questions. Available at https://www.lboro.ac.uk/departments/mlsc/studentresources/helmworkbooks/

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Colin Beveridge’s blog, "Flying Colours Maths" which I originally discovered because of a post explaining how to find all the invariant lines under a matrix transformation (https://www.flyingcoloursmaths.co.uk/askunclecolininvariantlines/)

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Adrian Banner’s \sayThe Calculus Lifesaver (the author can sure draw)

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Advanced Problems in Mathematics: Preparing for University https://www.openbookpublishers.com/product/342