Theorem lt2sqd

Description: The square function on nonnegative reals is strictly monotonic. (Contributed by Mario Carneiro, 28-May-2016)

	Ref	Expression
Hypotheses	resqcld.1	$$⊢\phi \to A\in ℝ$$
	lt2sqd.2	$$⊢\phi \to B\in ℝ$$
	lt2sqd.3	$$⊢\phi \to 0\le A$$
	lt2sqd.4	$$⊢\phi \to 0\le B$$
Assertion	lt2sqd	$$⊢\phi \to \left(A<B\leftrightarrow A^2<B^2\right)$$

Proof

Step	Hyp	Ref	Expression
1		resqcld.1	$$⊢\phi \to A\in ℝ$$
2		lt2sqd.2	$$⊢\phi \to B\in ℝ$$
3		lt2sqd.3	$$⊢\phi \to 0\le A$$
4		lt2sqd.4	$$⊢\phi \to 0\le B$$
5		lt2sq	$$⊢\left(\left(A\in ℝ\wedge 0\le A\right)\wedge \left(B\in ℝ\wedge 0\le B\right)\right)\to \left(A<B\leftrightarrow A^2<B^2\right)$$
6	1 3 2 4 5	syl22anc	$$⊢\phi \to \left(A<B\leftrightarrow A^2<B^2\right)$$