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Difference and Quotient Identities in ℝ

Elementary identities for differences and quotients of real numbers, the comparison of two reals through their difference, and the signs and absolute values of reciprocals and quotients, stated for ℝ in the form the calculus proofs use them.

Statement

In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let x,y,h,a,b,c,m∈Rx,y,h,a,b,c,m\in\mathbb{R}. Negatives and differences are as in Negatives, Differences, Reciprocals and Quotients (i), reciprocals and quotients as in Negatives, Differences, Reciprocals and Quotients (ii), absolute values as in Absolute Value in an Ordered Field (i), and << is the strict relation of the order ≤\le of R\mathbb{R}.

x−y=x+(−y)x-y=x+(-y); (x+h)−x=h(x+h)-x=h; x−(x−h)=hx-(x-h)=h; x−x=0x-x=0; −(x−y)=y−x-(x-y)=y-x; and x<yx<y if and only if 0<y−x0<y-x.

If b≠0b\neq0, then x/b=x⋅b−1x/b=x\cdot b^{-1}; (x/b)⋅b=x(x/b)\cdot b=x and b⋅(x/b)=xb\cdot(x/b)=x; (a−m⋅b)/b=a/b−m(a-m\cdot b)/b=a/b-m; and (a−c)/b=a/b−c/b(a-c)/b=a/b-c/b.

If 0<x0<x and 0<b0<b, then 0<x/b0<x/b. If b<0b<0, then b≠0b\neq0 and b−1<0b^{-1}<0. If b≠0b\neq0, then ∣b∣≠0|b|\neq0, ∣b−1∣=∣b∣−1|b^{-1}|=|b|^{-1} and ∣x/b∣=∣x∣/∣b∣|x/b|=|x|/|b|.

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