等差数列an的前7项的和为42;Sn=na1+[n(n-1)d]/2==>S7=7*a1+7*(7-1)*d/2=7a1+21d=42---(1);S9=9*a1+(9-1)*9d/2=9a1+36d;a4=a1+(4-1)d=a1+3d;S9-a4=57;==>9a1+36d-a1-3d=57==>8a1+33d=57;----(2)联立(1)(2)解得:d=1;a1=3;
