1 the tenth term of an arithmetic series is 67 and the sum of the first twenty terms is 1280.find the common difference
问题描述：
1 the tenth term of an arithmetic series is 67 and the sum of the first twenty terms is 1280.find the common difference ,d and the first term a .
2 if the first term of an arithmetic sequence is 3 and the hundredth tern is 307,then find the sum of the first 100 terms.
3 an arithmetic sequence has thirteen terms whose sum is 143.the third term is 5.find the first term.
答
a10=67,s20=1280 20*(a1+a20)/2=1280
-->a1+a20=a1+a1+19d=128
a10=(a1+a19)/2=67 --> (128-d)/2=67
-->d=-6
a=121
2.a1=3 a100=307 -->s100=(3+307)*100/2=15500
3.s13=143 a3=5
a1=5-2d a13=5+10d
(10+8d)*13/2=143 --> d=1.5
a1=2