7. （2023•临高县模拟）给定下列四个命题：命题①$a>b$，$c>d\Rightarrow a-c>b-d$；命题②：$a>b\Rightarrow (\frac{1}{2})^{a}<(\frac{1}{2})^{b}$；命题③：$\left\{\begin{array}{l}{0<a<1}\\ {2<b<3}\end{array}\right.<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mo>⇒</mo></mrow><annotation encoding="application/x-tex">\Rightarrow</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.3669em;"></span><span class="mrel">⇒</span></span></span></span></span>\left\{\begin{array}{l}{2<a+b<4}\\ {0<ab<3}\end{array}\right.$；命题④：$a<b<0\Rightarrow \frac{b}{a}<\frac{a}{b}$．其中真命题的个数是$($　　$)$