a1+a101=a3+a99=200 S101=101×(或橡毁a1+a101)除衫备2=10100
a15+a12+a9+a6=2(a1+a20) S20=20×(如返a1+a20)÷2=100
1.s101=(a3+a99)*101/2=10100
2.s20=(a15+a12+a9+a6)*20/4=100
3.an=2^(n-1)
1 .10100 2.100 3.an=2的n-1 次方
1,10100
2,100
3,2的n-1次
S101=101a51=101(a3+a99)/梁尺没橡纳困磨2=10100
s20=10(a1+a20)=5(a15+a6+a12+a9)=100
a1=1 a2=2 a3=4 or a1=4 a2=2 a3=1
an=2^(n-1) or an=2^(3-n)