[tex]\displaystyle\bf\\Orice~numar~natural~cuprins~intre~700~si~800~are~prima~cifra~7.\\Acest~numar~este~de~forma~\overline{7xy}\\\\Cu~alte~cuvunte:\\Daca~~700<n<800~~atunci~n=\overline{7xy}\\\\Cifrele:~7;~x;~y~sunt~consecutive.\\\\x=7+1=8\\y=x+1=8+1=9\\\\\implies~~\boxed{\bf n=789}[/tex]
