a2+a3=a1+d+a1+2d=2a1+3d=13
解得：d=3
又a4+a5+a6= (a1+a2+a3)+9d=15+27=42能说清楚一点吗，我有点看不懂a2=a1+da3=a1+2da4=a1+3da5=a2+3da6=a3+3d