①a1+a1+2d+a1+6d=28（a1+2d）（a1+2d）=a1（a1+6d）a1a1+4a1d+4dd=a1a1＋6a1d4dd=2a1da1=2da1=4，d=2②an=a1+（n-1）d＝4+2（n－1）=2n+2
（1）a1=4，d=2（2）An=2＋n
