帮个忙,帮我出50道人教版初三物理密度填空题!十万火急一定填空题哦

Question

帮个忙,帮我出50道人教版初三物理密度填空题!十万火急
一定填空题哦

Answer 1

an是方程(n^2+n)x^2+(n^2+n-1)x-1=0的根 ,解得 an=1/(n^2+n) 或an=-1(舍)
即an=1/[n*(n+1)] =1/n-1/(n+1)
Sn=a1+a2+a3+.+an
=1-1/2+1/2-1/3+1/3-.+1/n-1/(n+1)
=1-1/(n+1)
=n/(n+1)
limSn=1