题意说明,当n→∞时,必然有(r/(r+1))^n趋近于0.
则只能有|r/(r+1)|＜1
→ -1＜r/(r+1)=1-1/(r+1)＜1
→ 0＜1/(r+1)＜2
→ r+1＞1/2
→ r＞-1/2