Theorem rpmtmip

Description: "Minus times minus is plus", see also nnmtmip , holds for positive reals, too (formalized to "The product of two negative reals is a positive real"). "The reason for this" in this case is that ( -u A x. -u B ) = ( A x. B ) for all complex numbers A and B because of mul2neg , A and B are complex numbers because of rpcn , and ( A x. B ) e. RR+ because of rpmulcl . Note that the opposites -u A and -u B of the positive reals A and B are negative reals. (Contributed by AV, 23-Dec-2022)

	Ref	Expression
Assertion	rpmtmip	|- ( ( A e. RR+ /\ B e. RR+ ) -> ( -u A x. -u B ) e. RR+ )

Proof

Step	Hyp	Ref	Expression
1		rpcn	|- ( A e. RR+ -> A e. CC )
2		rpcn	|- ( B e. RR+ -> B e. CC )
3		mul2neg	|- ( ( A e. CC /\ B e. CC ) -> ( -u A x. -u B ) = ( A x. B ) )
4	1 2 3	syl2an	|- ( ( A e. RR+ /\ B e. RR+ ) -> ( -u A x. -u B ) = ( A x. B ) )
5		rpmulcl	|- ( ( A e. RR+ /\ B e. RR+ ) -> ( A x. B ) e. RR+ )
6	4 5	eqeltrd	|- ( ( A e. RR+ /\ B e. RR+ ) -> ( -u A x. -u B ) e. RR+ )