Răspuns:
Explicație pas cu pas:
foloseşti formula lui Heron:
[tex]S=\sqrt{p(p-a)(p-b)(p-c)} , unde, p ,este ,semiperimetru[/tex]
p=(5+7+9)/2=21/2
[tex]S=\sqrt{\frac{21}{2}( \frac{21}{2}-5)(\frac{21}{2}-7)(\frac{21}{2}-9)} =[/tex]
[tex]=\sqrt{\frac{21}{2}*\frac{11}{2} *\frac{7}{2} *\frac{3}{2} } =\frac{\sqrt{4851} }{4} =\frac{21\sqrt{11} }{4}[/tex]