mathworld.wolfram.com
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MathWorld is a comprehensive online mathematics encyclopedia from Wolfram Research. It offers in-depth articles with definitions, theorems, formulas, proofs, and illustrative diagrams across math topics, including statistics concepts like variance and covariance.
More resources on Variance & Covariance
Seeing Theory - Uncertainty
Visual interactive on probability distributions and variance
Khan Academy - Variance
Free interactive lessons on variance and covariance
Probability - The Science of Uncertainty and Data
Build foundational knowledge of data science with this introduction to probabilistic models, including random processes and the basic elements of statistical inference -- Part of the MITx MicroMasters program in Statistics and Data Science.
Introduction to Probability
Coursera course: An Intuitive Introduction to Probability
probabilitycourse.com
ProbabilityCourse.com is an online resource offering beginner-friendly tutorials on probability and statistics. It covers topics such as random variables, expectation, variance, and covariance with step-by-step explanations, worked examples, and practice problems.
ocw.mit.edu
MIT OpenCourseWare is a free, comprehensive repository of MIT course materials—lecture notes, assignments, exams, and videos—covering mathematics and statistics, including probability topics like variance and covariance, for self-paced study.