由等差中项,这是等差数列,a1=8,d=-2,a5=0,a6=-2,.an=10-2n
所以 当n5时,Sn=(a1+a2+a3+a4+a5)- (a6+a7+.+an)
=20-(-2+10-2n)*(n-5)/2
=n^2 -9n +40