Khan Academy: Negative numbers
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Khan Academy arithmetic unit of short videos and graded practice exercises on negative numbers: locating them on the number line, comparing them, and adding, subtracting, multiplying and dividing signed integers. Builds the sign fluency required before starting pre-algebra.
More resources on Integers
Negative Math: How Mathematical Rules Can Be Positively Bent
A historian of mathematics at UT Austin traces how negative numbers were resisted for centuries and shows that 'minus times minus equals plus' is a chosen convention, then constructs a consistent alternative algebra where it does not hold. Requires only high-school algebra but high conceptual load, hence 'advanced'.
NRICH: Positive and Negative Numbers (task collection)
A tagged collection of 26 investigations and games on positive and negative numbers from Cambridge's Millennium Mathematics Project. Problems such as Strange Bank Account and Consecutive Negative Numbers force reasoning about sign and order rather than rule recall.
OpenStax Prealgebra 2e — Chapter 3: Introduction to Integers
A free open-access chapter from Rice University's OpenStax Prealgebra, covering the number line, opposites, absolute value, and the four operations on integers. Worked examples and several hundred graded exercises with answers let you check your own work.
mathsisfun.com
MathIsFun.com is a beginner-friendly math site that explains concepts with clear, step-by-step lessons and interactive activities. It covers topics from algebra and linear equations to geometry, fractions, data, and more, with practice problems, worksheets, and quizzes.