. . . holds.¹

In mathematics, a double negative really is positive.

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. . . Laws:²

Note that division is not commutative: $a/b \ne b/a$ ! Neither is subtraction, for that matter: $a-b \ne b-a$ . The Commutative Law for multiplication, $ab=ba$ , holds for ordinary numbers (real and imaginary) but it does not necessarily hold for all the mathematical "things" for which some form of "multiplication" is defined! For instance, the group of rotation operators in 3-dimensional space is not commutative - think about making two successive rotations of a rigid object about perpendicular axes in different order and you will see that the final result is different! This seemingly obscure property turns out to have fundamental significance.

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. . . consistent³

In Mathematics we never worry about such things; all our symbols represent pure numbers; but in Physics we usually have to express the value of some physical quantity in units which make sense and are consistent with the units of other physical quantities symbolized in the same equation!

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. . . generally⁴

The $\pm$ symbol means that both signs (+ and $-$ ) should represent legitimate answers.

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