[求助] 几道数学题

Henrik

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 two consecutive terms is negative. What is the number of variations in sign for the sequences 1, -3, 2, 5, -4, -6?

3        怎么读不懂是什么意思了? 应该怎样理解题意?

 

 

The rate of a certain chemical reaction is directly proportional to the concentration of chemical A present and inversely proportional to the concentration of the chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

40 % increase.
                

y=A'2/ B 但是要怎么去想 A的变化?

 

 

 

If n is a positive integer and r is the remainder when 4 + 7n is divided by 3, what is the value of r?

1)      n + 1 is divisible by 3

2)      n>20.

A  

4+7n is odd, with n+1 is divisible by 3, n could only be 5. Correct?
                

 

 

If the operation # is one of the four arithmetic operations addition, subtraction, multiplication, and division, is (6#2)#4=6#(2#4)?

1)      3#2>3

2)      3#1=3.  3*1=3   3/1=3. Not sufficient.

A

从1)里可以判断#是加或乘, 但是不能确定到底是什么. 选A不是违背了答案唯一的原则了吗?

 

 

谢谢了

stavan

对于一个有限的由非零数字组成的数列, the number of variations in sign被定义为: 该数列中两个连续的项相乘结果为负数的组数, 那这就简单了...1, -3, 2, 5, -4, -6中, 有3组的乘积是负数, 所以结果就是3

 

这个题你抄的题目有误，应该是A的平方，

因为没有涉及其他因素的变化，所以可以简单设为 y=(A^2)/B, B增加100%， 那么就是说（A^2)也要增加100%才能保证y不变

(A^2)增加100%, 就是A增加约为40%, 如果这么说不好理解, 那我们把B增加100%理解为2B, 为保证不变,就是说(A^2)要相应增加成2(A^2), A要增加到√2A, √2=1.414, 那么A增加41.4%, 得出答案约为增加40%

 

没看懂楼主具体要问什么...

题目说, 4+7n除以3的余数为r, 问r=?

条件1, n+1可以被3整除, 那么7n+7必然也可以被3整除, 那么7n+4=(7n+7)-3也必然可以被3整除,r=0

条件2, n>20, 只能得出4+7n>144, 无法对r进行判断

所以选择A

 

不违背原则, 因为题目问(6#2)#4是否等于6#(2#4), 只要能回答,是或者否则为答案,

条件1判断出#为加或者乘, 无论是那个, 都可以得出(6#2)#4=6#(2#4), 所以是答案

条件2判断出#为乘或除, 是乘的时候(6#2)#4=6#(2#4), 是除法的时候(6#2)#4不等于6#(2#4), 无法判断

所以选择A, 楼主注意看清楚题目究竟问的什么, 如果题目问#是加减乘除的哪一个

那么根据1,2联立,可以得出#为乘, 但是题目问的是(6#2)#4=6#(2#4), 无论#代表什么, 只要能判断这个式子是否成立就可以了

Henrik

Thank you, Stavan. 现在明白了. 很常时间没有复习, 看的时候总有点晕.