Generally, no topic is specified. In geometry, r refers to the radius of a circle (if there are two circles or two radii, take the smaller one); In statistics, r is the correlation coefficient; There is Cn^r in the permutation and combination, and this R is to take R from N.

If it is R, it is a real number without explanation or special background. If it is geometry, it is radius; If it is statistics, R 2 is 1- sum of squares of residuals/sum of squares of total deviations.

If some of the derivatives involve physics, R generally stands for internal resistance.

In a word, R has many meanings, depending on the actual situation.