[tex]\displaystyle\bf\\Desenul~este~in~imaginea~atasata.\\\\Rezolvare:\\Diametrul~cercului~este~ipotenuza~triunghiului~dreptunghic.\\\\Dc=EF=\sqrt{DE^2+DF^2}=\sqrt{9^2+12^2}=\sqrt{81+144}=\sqrt{225}=15~cm\\\\Rc=\frac{Dc}{2}=\frac{15}{2}=7,\!5~cm\\\\Lungimea~cercului=Lc=2\pi Rc=2\pi \times7,\!5=15\pi~cm[/tex]
