请教数学3道~

vcheung · 发表于 2010-11-17 09:02:12

本人数学总是在坐标方面错得很厉害,基本但凡有坐标的题,都拿不准~在这里请教各位NN们三道数学题~

1. If k is a line in the xy-plane, what is the slope of k?
(1) the x-intercept of k is 2.
(2) the y-intercept of k is 3.
答案是C. 为什么?

2. In the xy-plane, line k has positive slope and x-intercept 4. If the area of the triangle formed by the line k and the two axes is 12, what is the y-intercept of link k?

答案是-6. 为什么不是6呢?

3. In a certain furniture stone, each week Nancy earns a salary of $240 plus 5% of the amount of her total sales that exceeds $800 for the week. If Nancy earned a total of $450 one week, what were her total sales that week?

答案是$5000. 为什么不是$4200呢?

再次谢谢各位NN指教~~~~

vcheung · 发表于 2010-11-17 09:24:39

请各位NN帮帮忙哇~~~

pdszhanggege · 发表于 2010-11-17 09:40:23

1,只有知道了一条线上的两个坐标，才可以求出该线的斜率~所以第一题（1）和（2）together才可以知道K线的斜率
2，第二题没有明白，是link k ???什么意思？？是line k 吗？？
position slope ？？是positive slope吗？
3，她的总工资是240加上总的sales超过800的那部分的5%，用（450—240)/0.05+800 即可

vcheung · 发表于 2010-11-17 09:53:48

1,只有知道了一条线上的两个坐标，才可以求出该线的斜率~所以第一题（1）和（2）together才可以知道K线的斜率
2，第二题没有明白，是link k ???什么意思？？是line k 吗？？
position slope ？？是positive slope吗？
3，她的总工资是240加上总的sales超过800的那部分的5%，用（450—240)/0.05+800 即可

-- by 会员 pdszhanggege (2010/11/17 9:40:23)

非常感谢MM的解答~~~是line k, positive slope....不好意思,已经更改~

crystalsnowwu · 发表于 2010-11-17 10:25:29

1,只有知道了一条线上的两个坐标，才可以求出该线的斜率~所以第一题（1）和（2）together才可以知道K线的斜率
2，第二题没有明白，是link k ???什么意思？？是line k 吗？？
position slope ？？是positive slope吗？
3，她的总工资是240加上总的sales超过800的那部分的5%，用（450—240)/0.05+800 即可

-- by 会员 pdszhanggege (2010/11/17 9:40:23)

Q#1里面给的是x, y轴上的截距，可是不是坐标呀。这样不就可能有4条不同的直线了么。。我觉得是选E。。求解答= =

vcheung · 发表于 2010-11-17 10:43:21

1,只有知道了一条线上的两个坐标，才可以求出该线的斜率~所以第一题（1）和（2）together才可以知道K线的斜率
2，第二题没有明白，是link k ???什么意思？？是line k 吗？？
position slope ？？是positive slope吗？
3，她的总工资是240加上总的sales超过800的那部分的5%，用（450—240)/0.05+800 即可

-- by 会员 pdszhanggege (2010/11/17 9:40:23)

Q#1里面给的是x, y轴上的截距，可是不是坐标呀。这样不就可能有4条不同的直线了么。。我觉得是选E。。求解答= =

-- by 会员 crystalsnowwu (2010/11/17 10:25:29)

我刚开始也是选E来着~~~答案选C,然后结合pdzhanggege 的答复,才明白选C的道理~~~

sdcar2010 · 发表于 2010-11-17 10:56:37

In the xy-plane, line k has positive slope and x-intercept 4. If the area of the triangle formed by the line k and the two axes is 12, what is the y-intercept of link k?

- - - - -

You need a negative slope in order to form a triange with two axises.

vcheung · 发表于 2010-11-17 11:13:21

In the xy-plane, line k has positive slope and x-intercept 4. If the area of the triangle formed by the line k and the two axes is 12, what is the y-intercept of link k?

- - - - -

You need a negative slope in order to form a triange with two axises.

-- by 会员 sdcar2010 (2010/11/17 10:56:37)

能具体点吗?谢谢~

sdcar2010 · 发表于 2010-11-17 11:47:44

Sorry. I got confused.

If the triangle is formed by three points : (0,0), (4,0) and (0,6), then the slope of line k is negative.

If the triangle is formed by three points : (0,0), (4,0) and (0,-6), then the slope of line k is positive (as described by the stimulus).

vcheung · 发表于 2010-11-17 11:53:29

Sorry. I got confused.

If the triangle is formed by three points : (0,0), (4,0) and (0,6), then the slope of line k is negative.

If the triangle is formed by three points : (0,0), (4,0) and (0,-6), then the slope of line k is positive (as described by the stimulus).

-- by 会员 sdcar2010 (2010/11/17 11:47:44)

根据题意,line k has positive slope, 再结合你的总结,所以是-6. 谢谢哈~

请教数学3道~

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