Question

设{an}是公差为正数的等差数列,若a1+a2+a3=15,a1a2a3=80,...

设{an}是公差为正数的等差数列,若a1+a2+a3=15,a1a2a3=80,则a11+a12+a13=( )A.120B.105C.90D.75

Answer 1

{an}是公差为正数的等差数列,
∵a1+a2+a3=15,a1a2a3=80,
∴a2=5,
∴a1a3=(5-d)(5+d)=16,
∴d=3,a12=a2+10d=35
∴a11+a12+a13=105
故选B.