解:當(dāng)??$n=0$??時(shí)??$， n2+n=0；$??當(dāng)??$n=1$??時(shí)??$， n2+n=2；$??當(dāng)??$n=2$??時(shí)??$， n2+n=6，$??
當(dāng)??$n=3$??時(shí)??$， n2+n=12；$??當(dāng)??$n=4$??時(shí)??$， n2+n=20$??
所以當(dāng)??$n=0， 1， 2， 3， 4$??時(shí)??$， n2+n$??的值是偶數(shù)
??$n2+n=n(n+1) $??
所以對(duì)于所有自然數(shù)??$， n2+n$??的值都是偶數(shù)
