an=a1+(n-1)d
n=((an-a1)/d)+1
=((3n+7-1)/3)+1
=n+3
1+4+7+10+.+(3n+4)+(3n+7)
=(3n+7+1)(n+3)/2
=(3n+8)(n+3)/2一共有多少项？项数怎么求an=a1+(n-1)dn=((an-a1)/d)+1