Question

已知等差数列{an}的前13项之和为39,则a6+a7+a8等于

Answer 1

a1 + a2 + ......+ a12 + a13= (a1+a13) + (a2 +a12) + ......+ (a5+a9) + (a6 +a8) + a7= 2a7 + 2a7 + ...... + 2a7 +2a7 + a7= 2a7 * 6 + a7= 13a7= 39 ==> 13a7 = 39==> a7 = 3 a6 + a7 + a8 = (a6 +a8) +a7= 2a7 + a7= 3a7= 3 * 3=9

Answer 2

等于9…等差数列前十三项等于13倍的a7…得出a7等于3…让你球的三项和等于3倍的a7…所以等于9

Answer 3

39/13=3a7=3a6+a7+a8=3*3=9