项数为奇数,设为S奇=a1+a3...a(2k+1)S偶=a2+a4...+a2kS奇-S偶=a1+(a3-a2)+(a5-a4)+...=a1+kd=a(k+1)为中间项即44-33=11Sn=a(k+1)*n则n=Sn/a(k+1)=(44+33)/11=7
