0, b>0\\\\S=ab\\a:b=3:4\\\\ \left \{ {{ab=300} \atop {a:b=3:4}} \right. \\\\ \left \{ {{a=300/b} \atop {a=3b/4}} \right. \\\\300/b=3b/4\\3b^2=1200|:3\\b^2=400\\b= \sqrt{400}=20\\\\a=300/20=15\\\\P=2(15+20)=2*35=70 " alt="P=2(a+b), a>0, b>0\\\\S=ab\\a:b=3:4\\\\ \left \{ {{ab=300} \atop {a:b=3:4}} \right. \\\\ \left \{ {{a=300/b} \atop {a=3b/4}} \right. \\\\300/b=3b/4\\3b^2=1200|:3\\b^2=400\\b= \sqrt{400}=20\\\\a=300/20=15\\\\P=2(15+20)=2*35=70 " align="absmiddle" class="latex-formula">