两道小白prep数学~请帮忙一下吧~

chuyou1989

1，The sequence a₁, a₂, a₃, . . . , a_n of nintegers is such that a_k = k if k is odd and a_k = -a_{k - 1} if k is even.Is the sum of the terms in the sequence positive?



(1)n is odd.



(2)a_n is positive.







答案为D



确实是都能算出来啊。但是所有的结果都带有N吧~难道N算是已知数吗？或者是单一确定值吗？

Ellen can purchase a certain computer at a local store at the price of p dollars and pay a 6 percent sales tax.Alternatively, Ellen can purchase the same computer from a catalog for a total of q dollars, including all taxes and shipping costs.Will it cost more for Ellen to purchase the computer from the local store than from the catalog?



(1) q - p < 50



(2) q = 1,150



答案为C



不理解啊，求的应该是q-1.06p是否大于0吧？但是（1）（2）结合算出来应该有时大于0，有时小于0.吧。或者，我漏了什么关键码？

zafteria

两道题都没问题啊
1. (2) can conclude n is odd hence the same as (1) ,so choose D
2. (1) and (2) can canclude that p>q-50=1100, hence 1.06p>1166>q ,clearly answer the queson. So choose C

cherishzhao

第一题：

算出来的结果是都带n的，这样就可以判断正负啦，因为n是正是负总知道的呀~lz可以自己举个例子算一下


第二题：


比较q-1.06p和0的大小，因为q=1150,q-p<50
所以p>1000


q-1.06p=q-p-0.06p<50-0.06p<50-1000*0.06<0


就比较出来了~

chuyou1989

明白了~
谢谢楼上两位大牛