已知等差数列{a小n},a2加a3加a7加a11加a12=45,则S13=?急

已知等差数列{a小n},a2加a3加a7加a11加a12=45,则S13=?急
已知等差数列{a小n},a2加a3加a7加a11加a12=45,则S13=?急

已知等差数列{a小n},a2加a3加a7加a11加a12=45,则S13=?急
因为a2+a3+a7+a11+a12=45
且a2+a12=2a7,a3+a11=2a7
所以5a7=45
故a7=9
所以S13=13(a1+a13)/2=13*a7=13*9=117

提示：等差数列性质：若mnpq都是正整数，且 m+n=p+q,
则：am+an=ap+aq