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?
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![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?](/uploads/image/z/8830671-15-1.jpg?t=%E5%B7%B2%E7%9F%A5%E6%95%B0%E5%88%97%EF%B9%9Ban%EF%B9%9C%E4%B8%AD%2Ca3%3D2%2Ca7%3D1%2C%E5%8F%88%E6%95%B0%E5%88%97%EF%B9%9B1%2F%EF%B9%99an%2B1%EF%B9%9A%EF%B9%9C%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2C%E5%88%99a11%E7%AD%89%E4%BA%8E%3F)
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