Изучаю математику можно сказать с нуля.
В одной книжке пишут:
Цитата:
... the absolute value of a real number is a non-negative real number. If a given number is zero or positive, its absolute value is the number itself. If a number is negative, its absolute value is the opposite of the number; that is, its absolute value is a positive number. We summarize in the following definition
If
![$r$ $r$](https://dxdy-01.korotkov.co.uk/f/8/9/f/89f2e0d2d24bcf44db73aab8fc03252c82.png)
is a real number which is positive or zero, then
![$\lvert r \rvert = r$ $\lvert r \rvert = r$](https://dxdy-03.korotkov.co.uk/f/6/3/a/63aea59400c2963c58e95bb97262e17f82.png)
; if
![$r$ $r$](https://dxdy-01.korotkov.co.uk/f/8/9/f/89f2e0d2d24bcf44db73aab8fc03252c82.png)
is a negative number, then
![$\lvert r \rvert = -r$ $\lvert r \rvert = -r$](https://dxdy-04.korotkov.co.uk/f/f/e/f/fef051ab606fe002fd13484040dc3dde82.png)
может я чего-то не догоняю,
по моему, с учетом предложения "If a number is negative, its absolute value is the opposite of the number; that is, its absolute value is a positive number" должно быть
![$\lvert -r \rvert = r$ $\lvert -r \rvert = r$](https://dxdy-03.korotkov.co.uk/f/2/9/2/2929f855076adf95496a3ce92851c7c782.png)