When you choose to send one of our free E-cards, you save paper and give friends and family the opportunity to enjoy the wonder of nature. Celebrate the beauty of our natural world and send an E-card
在等差数列{an}中，当a1=a5(r≠s)时，数列{an}必定是常数列，然而在等比数列{an}中，对某些正整数r,s(r≠s)，当ar=as时，{an}不是常数列的一个例子是_________.
补写出下列名篇名句中的空缺部分。 古代诗文中,有许多描写丝竹乐器演奏的名句,如李贺《李凭箜篌引》中的“ ，芙蓉泣露香兰笑”；白居易
You should put on the notices ________ all the people may see them. A. where B. in which C. at D. for them
设f(n)=+…+(n∈N*),那么f(n+1)-f(n)等于（ ） A. B.C. D.
完形填空 Hello! My name is Dave Brown. My 1 name is Dave. I play sports every day and I am very 2 . I eat a lot of fruit and 3 . For fruit, I like apples, oranges, pears and 4 . For vegetables,
The girl wanted to play in the park, but her mother told her ____. A. not to B. not to do C. to not do it D. to not
如图，是直线上一点，为任一条射线， 平分，平分． （1）指出图中与的补角； （2）试说明与具有怎样的数量关系．
