Don't worry about the "$r$"'s. Let them hang along for the ride. Think of $r$ as just a specific number, if you like.
Let's start with the left hand side of your identity: $\tag{1} (\color{maroon}2r\color{maroon}{\sin A\cos A})^2+r^2(\color{darkgreen}{\cos^2 A-\sin^2 A})^2 $ and apply the double angle identities you've noted. Replacing, as we may, in $(1)$ the expression $\color{maroon}{2\sin A\cos A}$ with $\sin (2A)$ and the expression $\color{darkgreen}{\cos^2A-\sin^2A}$ with $\cos( 2A)$ we obtain $\tag{2} \bigl(r\sin( 2A)\bigr)^2 +r^2\bigl(\cos(2A)\bigr)^2 $
Using the rule $(ab)^2=a^2b^2$ and the notations $(\sin x)^2=\sin^2x$ and $(\cos x)^2=\cos^2 x$, we can write $(2)$ as $ \tag{3} r^2\sin^2(2A)+r^2\cos^2(2A). $ Next, let's factor the $r^2$ term out. We can write $(3)$ as $\tag{4} r^2\bigl(\sin^2 (2A)+\cos^2(2A)\bigr). $ But Pythagoras tell us that $\sin^2 (2A)+\cos^2 (2A)=1$; thus the expression in $(4)$ is simply $r^2$, which is what you wanted to show.