数学求解，急！大牛快来帮忙

rabbit861013

这四道题目是在今天的模拟卷子上碰到的，大牛们快来帮忙，谢谢！

1.For a finite sequence of nonzero numbers, the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence for which the product of the consecutive terms is negative.What is the number of variations in sign for the sequence 1,-3.2.5.-4.-6?

答案是3，可是我认为答案是4 ，因为1-（-3）=4

2.Malik's recipe for 4 servings of a certain dish requires 3/2 cups of pasta.Accordingto the recipe, What is the number of cups of pasta next time he prepares this dish?

(1)The next time he prepares this dish, Malik will make half as many servings as he did the next time he prepared the dish

(2)Malik used 6 cups of pasta the last time he prepared this dish

答案是C,可是我认为应当选择A

3.For a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years. At the end of the 6th year, the tree was 1/5 taller than it was at the end of the 4th year.By how many feet did the height of the tree increase each year?答案是2/3,可是我算了几遍答案都是1/10.用的是等差数列。

4.For a certain examination,a score of 58 was 2 standard deviation below the mean,and a score of 98 was 3 standard deviation above the mean.What was the mean score of the examination?

答案是74,可是我算不出这个答案，请大牛指教

alan722

1.题目的意思是这组数字中，相邻的2个相乘，结果为负数，就属于所谓的variations in sign，数下来只有3组，（1，-3），（-3，2），（5.-4）

2.条件一只说了下一次做前一次一半的量，显然得不出答案

3.注意增长的是1/5，是个分数，不是1/5feet，答案是2/3没错

4.简单的二元二次方程，跟中位数，标准方差的概念无关。设中位数X，平均数Y，题目意思 Y-2X=58 ; Y+3X=98

 

rabbit861013

感谢大侠

rabbit861013

请问第三题中a4 与a6 的关系式是写成a6=1/5(1+a4)吗？ 我算出来答案是4/7 ?

rabbit861013

顺便再问一题

If a certain sample of data has a mean of 20.0and a standard deviation of 3.0. Which of the following values is more than 2.5 standard deviation from the mean?

rabbit861013

请问大牛在吗？帮帮忙