解析an-a(n-1)=10-2n-10+2(n-1)=10-2n-10+2n-2=-2所以数列是首项8公差-2的等差数列所以Sn=a1n+n(n-1)d/2=8n-n(n-1)=8n-n^2+n=9n-n^2=-n^2+9n=-(n^2-9n)=-[(n-9/2)^2-81/4]=-(n-9/2)^2+81/4所以n=5或n=4时取得最大值2...