设Sn是等差数列an的前n项和a5=2,an-4=30Sn=136,n>=5求n?

问题描述:

设Sn是等差数列an的前n项和a5=2,an-4=30Sn=136,n>=5求n?

Sn=n/2*(a1+an)=n/2*(a5+an-4)=136 n=2*136/32 带进正确的值求出