∵a2，a4，a7成等比数列，∴（a1+3d）2=（a1+d）（a1+6d）．∴a1=3d，∴Sn=na1+ n(n?1) 2 d= n2+5n 2 d，∴ S10 S5 = 100+50 25+25 =3．故答案为：3．
