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