题目

设{an}是公差为正数的等差数列，若a1+a2+a3=15,a1a2a3=80,则a11+a12+a13等于（    ）A.120          B.105              C.90             D.75 答案：B解析:a1+a2+a3=153a2=15a2=5,a1a2a3=80(a2-d)a2(a2+d)=80,将a2=5代入，得d=3,从而a11+a12+a13=3a12=3(a2+10d)=3×(5+30)=105.

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