找出3,9,18,30的规律,并用这条规律说出第n个数是几
问题描述：
找出3,9,18,30的规律,并用这条规律说出第n个数是几
答
a2=9=3+6=a1+6
a3=18=9+9=a2+9
a4=30=18+12=a3+12
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an=a(n-1)+3(n-1)+3=a(n-1)+3n
a2+a3+a4+----+an=a1+a2+a3+---a(n-1)+(6+9+12+----+3n)
an=a1+(6+9+12+----+3n)
=3+6+9+12+-----3n
=3n(n+1)/2
第n个数是3n(n+1)/2