数列an+1=an2+an(n+1为脚标),a1=1,求证1/(a1+1)+1/(a2+1)+1/(a3+1)+...+

3个回答

  • 题目出错了,不等号方向反了,而且没有规定n的取值.

    证:

    假设当n=k(k∈N,且k≥1)时,ak>0,则当n=k+1时,a(k+1)=ak²+ak=ak(ak+1)

    ak>0 ak +1>0 a(k+1)>0

    k为任意正整数,因此an>0,即数列各项均>0

    a(n+1)=an²+an

    a(n+1)/an=an+1>1

    a(n+1)>an,数列为递增数列.

    a2=a1²+a1=1+1=2

    n≥2时,an≥2 1/an≤1/2

    a(n+1)=an²+an

    1/a(n+1)=1/[an²+an]=1/[an(an +1)]=1/an -1/(an +1)

    1/(an +1)=1/an -1/a(n+1)

    1/(a1+1)+1/(a2+1)+...+1/(an +1)

    =1/a1-1/a2+1/a2-1/a3+...+1/an -1/a(n+1)

    =1/a1 -1/a(n+1)

    =1 -1/a(n+1)

    n≥1,n+1≥2 a(n+1)≥2 0