已知数列﹛an﹜中,a3=2,a7=1,又数列﹛1/﹙an+1﹚﹜为等差数列,则a11等于?

已知数列﹛an﹜中,a3=2,a7=1,又数列﹛1/﹙an+1﹚﹜为等差数列,则a11等于?
已知数列﹛an﹜中,a3=2,a7=1,又数列﹛1/﹙an+1﹚﹜为等差数列,则a11等于?

已知数列﹛an﹜中,a3=2,a7=1,又数列﹛1/﹙an+1﹚﹜为等差数列,则a11等于?
令bn=1/﹙an+1﹚
则b3=1/3,b7=1/8
由{bn}是等差数列,有
b11+b3=2b7
故b11=2b7-b3=-1/12
又由b11=1/﹙a11+1﹚
解得a11=-13