已知等差数列{An}中,公差d大于0,其前n项和为Sn,且满足:a2a3=45,a1...答:a1+a4=a2+a3=14 a2×a3=45 且d>0 可知a2=5,a3=9 且d=4,a1=1 1.An=a1+(n-1)×d =4n-3 Sn=a1+n×(n-1)/2×d =2n^2-2n+1,7,已知等差数列{An}中,公差d大于0,其前n项和为Sn,且满足:a2a3=45,a1+a4=14.1.求数列{An}的通项公式;2.通过公式Bn=Sn/(n+c)构造一...
已知等差数列{an}的公差d>0,其前n项和为sn,满足S9<0,S10>0,则n=多少...答:Sn=n*(a1+an)/2 S9=9*(a1+a1+8d)/2<0,得a1+4d<0 S10=10*(a1+a1+9d)/2>0,得a1+4.5d>0 d>0,a1<0,则-4d>a1>-4.5d a1=-4d,an=-4d+(n-1)d=0, 得n=5 a1=-4.5d,an=-4.5d+(n-1)d=0, 得n=5.5 可知5<n<5.5时,即n=5时,Sn最小值....