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标题: 10.12 首战放狗 [打印本页]

作者: vieooo 时间: 2019-10-13 10:42
标题: 10.12 首战放狗
Q：50
1. 有一个八条道的操场，直的两条道是100miles，最内圈的两个半圆直径是64miles，每条道比里面的多1mile，问最外面的比最里面的长多少？我选的是16π
2. 定义【a】是等于或者小于a的最大整数，｛a｝=a-【a】，应该就是剩下的小数部分。0＜a＜b，问【b/(a+b)】+{(a+b)/b}的值是多少？我选的a/b
3. 一道ds题，一种热狗有三种肉食选择，三种面包选择，三种cheese选择，第四个步骤是加不加芥末，或者加不加sauce，问几个特定组合的排列方式？记不清了
4. 问(3/4-2/9)-2/3和3/4-(2/9-2/3)的差距，我选的3/4less
5. 几何题，一个等腰直角三角形组合一个长方形，给了等腰直角三角形的斜边28，有一个角是30°，然后问一个边长，很简单，看了就能做。我选的14（根号6-根号2）
6. ds，20，30，x三个数，问算术平均数比极差大吗？1）x小于10；2）x大于5
其他的记起来再补充吧

月读：
1. 第一篇巨难，讲的是sand dune 的形成，怎么成堆的，怎么移动的，然后还有一些研究，生词太多没太读懂，建议考古，篇幅也很长。
2. 讲建大坝的利弊吧。
p1：讲拆了大坝会让河水变清澈，让河水降温；
p2：拆了大坝会让大坝底下的有毒物质和淤泥冲到下游，拥堵河道；大坝也能阻挡nonnative物种入侵道下游，从而保护下游生态
p3：讲大坝会阻挡有些本地物种向上游迁徙繁衍之类的。。这个挺简单的，问题记不清了，看到这个我觉得分可能不会太高了。

坐稳：
财政部长给council写信：建议全市民众一起fund一个luxury hotel near the convention.好处一是外来的游客来参观会买goods和sevice，然后游客的消费税越高，要求本地民众贡献给税收的就越低；好处二是游客会更享受豪华房间，消费也会比住inexpensive的宾馆多，这样促进税收增加。
我选的是自己理解的比较好写的点，无端假设，无因果联系。

哎，分数比较尴尬，后面再看看要不要刷一次，还要考雅思。别的不说，背单词大概永远是学一门语言的王道选择吧；前辈推荐的方法我都没咋试验，时间不太够，只做了OG，大致理解gmac出题的套路，长难句的理解速度还是需要提高的。各位加油！

作者: 弱鸡小梁 时间: 2019-10-13 12:15
感谢分享！

作者: billyisfragile! 时间: 2019-10-13 12:25

第一篇巨难，讲的是sand dune 的形成，怎么成堆的，怎么移动的，然后还有一些研究，生词太多没太读懂，建议考古，篇幅也很长。

Squeaking sand produces sounds with very high frequencies—between 500 and 2,500 hertz, lasting less than a quarter of a second. The peals are musically pure, often containing four or ﬁve harmonic overtones. Booming sand makes louder, low-frequency sounds of 50 to 300 hertz, which may last as long as 15 minutes in larger dunes (although typically they last for seconds or less). In addition, they are rather noisy, containing a multitude of nearby frequencies. Booms have never been observed to contain more than one harmonic of the fundamental tone.

These dramatic differences once led to a consensus that although both types of sand produce acoustic emissions, the ways in which they do so must be substantially different. In the late 1970s, however, Peter K. Haff, then at the California Institute of Technology, produced squeaks in booming sand, suggesting a closer connection between the two.

Both kinds of sand must be displaced to make sounds. Walking on some sand, for example, forces the sand underfoot to move down and out, producing squeaks. In the case of booming sand, displacement occurs during avalanches.

It is within the avalanche that sound begins and where the answers must be hiding. Before an avalanche can occur, winds must build a dune up to a certain angle, usually about 35 degrees for dry desert sand. Once the angle is achieved, the sand on the leeward side of the dune begins to slump. Intact layers of sand slip over the layers below, like a sheared deck of cards. At the same time, the individual grains in the upper layers tumble over the grains underneath, momentarily falling into the spaces between them and bouncing out again to continue their downward journey. Their concerted up-and-down motion is believed to be the secret source of sound. Fully developed avalanches, in which sliding plates of sand remain intact for most of their motion, have the greatest acoustic output. In some places, where large amounts of sand are involved, booming can be heard up to 10 kilometers away.

The mysteries of the vibrations are many. To begin with, the multiple frequencies of booming sand are not well understood. In the 1970s David R. Criswell and his collaborators at the University of Houston found that each frequency seems to exhibit its own rise-and-fall time, independent of the others. Taken together, these frequencies can cover a fairly broad range, the width of which is determined by various factors. For example, Sand Mountain booms at roughly 50 to 80 hertz; sands at Korizo, Libya, drone at between 50 and 100 hertz; and in the Kalahari Desert of South Africa, the frequencies range from 130 to 300 hertz. Such output—presumably caused by multiple modes of vibration within the shearing plates—is often unmusical and jarring.

Because it is caused by large volumes of shearing sand, the roaring is also loud. In fact, sounds made by booming sand can be nearly deafening, and the vibrations causing them can be so intense that standing in their midst is nearly impossible.

A good place to start in exploring the vibrational properties of sand is with the grains themselves. The mean diameter of most sand grains, whether acoustically active or not, is about 300 microns. Usually the grains in a booming dune are very similar in size, especially near the leeward crest, where the sound most often originates; such uniformity allows for more efﬁcient shearing. Otherwise, the smaller grains impede the smooth motion of the larger ones.

Similar sizes do not alone allow sand to boom. On the contrary, the booming sands of Korizo and Gelf Kebib, also in Libya, feature an uncharacteristically broad range of particle sizes. Moreover, silent dune sand often contains grains somewhat similar to nearby booming sand.

Grains of booming sand also tend to have uncommonly smooth surfaces, with protrusions on the scale of mere microns. Booming dunes are often found at the downwind end of large sand sources; having bounced and rolled across the desert for long distances, the sand grains in these dunes are usually highly polished. Over time a grain can also be polished by repeated shifts within a moving dune. And squeaking sand as well tends to be exceptionally smooth.

Close inspection of Sand Mountain and Kalahari booming sand, however, reveals that not all grains are highly spherical or rounded. And in 1936 A. D. Lewis in Pretoria, South Africa, even claimed to have produced booming in the cubed grains of ordinary table salt. Conversely, spherical glass beads cannot be made to boom. These ﬁndings show that although smoothness and roundness are essential to producing sound, so is some degree of roughness.

Another important factor is humidity, because moisture can modify the friction between the grains or cause sand to clump together, thus precluding shearing. Sounds occur in those parts of the dune that dry the fastest. Precipitation may be rare in the desert, but dunes retain water with remarkable efﬁciency. Sand near the surface dries quickly, however, and sand around a dune’s crest tends to dry the fastest.

Near the leeward crest, the combination of smooth, well-sorted grains and lack of moisture leads to conditions more likely to produce sounds during shearing. And because wind usually deposits more sand closer to the top of the lee face, sand accumulates there faster than in lower regions, thereby slowly increasing the dune’s incline to where avalanches occur.

Typically, large platelike slabs of sand break off near the crest. In booming sand, these plates tend not to slow into loose ﬂows as they encounter gentler slopes. Instead their upper parts collapse or telescope violently into the lower parts. The plates’ eventual breakup is unusually turbulent.

Learning more about sound-producing sand has not been easy. Research has been hindered by the rarity of the phenomenon—especially booming sand —and the difﬁculty in reproducing sounds in laboratories. In addition, for years researchers did not clearly differentiate between booming and squeaking sands, making the early literature on the topic less than reliable.

作者: viivici 时间: 2019-10-13 12:33
感觉的确和99重库啦…

作者: -______-b 时间: 2019-10-13 22:40
财政部长给council写信：建议全市民众一起fund一个luxury hotel near the convention.好处一是外来的游客来参观会买goods和sevice，然后游客的消费税越高，要求本地民众贡献给税收的就越低；好处二是游客会更享受豪华房间，消费也会比住inexpensive的宾馆多，这样促进税收增加。

很神奇的是作文题跟我9.28考的一样

作者: vivivi123 时间: 2019-10-13 23:26
数学第四题不应该是4/3 less嘛还有第七题应该选A对嘛

作者: AnnieLinbaby 时间: 2019-10-14 00:42
楼主，几何题，一个等腰直角三角形组合一个长方形，给了等腰直角三角形的斜边28，有一个角是30°，然后问一个边长，很简单，看了就能做。我选的14（根号6-根号2）
这题。。没有图很难理解是怎么样的，请问能解释一下吗
谢谢啦！

作者: AnnieLinbaby 时间: 2019-10-14 02:20
. 有一个八条道的操场，直的两条道是100miles，最内圈的两个半圆直径是64miles，每条道比里面的多1mile，问最外面的比最里面的长多少？我选的是16π..楼主这题感觉跟9/9也重库了
但我没读懂题目，你的16pie哪里的？确定答案选项有pie在里面吗？
能不能分享一下思路
感谢！

作者: AnnieLinbaby 时间: 2019-10-14 02:34
问(3/4-2/9)-2/3和3/4-(2/9-2/3)的差距，我选的3/4less
楼主，怎么感觉
-2/3 - 2/3 -> -4/3 呀
为什么你的答案是-3/4呀！？

作者: vieooo 时间: 2019-10-14 10:13

bzy! 发表于 2019-10-13 12:25
Squeaking sand produces sounds with very high frequencies—between 500 and 2,500 hertz, lasting l ...

内容有些重叠，但好像不是这个

作者: vieooo 时间: 2019-10-14 10:14

vivivi123 发表于 2019-10-13 23:26
数学第四题不应该是4/3 less嘛还有第七题应该选A对嘛

我打错了，不好意思，是-4/3；我没写第七题哇

作者: vieooo 时间: 2019-10-14 10:15

AnnieLinbaby 发表于 2019-10-14 02:34
问(3/4-2/9)-2/3和3/4-(2/9-2/3)的差距，我选的3/4less
楼主，怎么感觉
-2/3 - 2/3 -> -4/3 呀

-4/3，我打错了

作者: vieooo 时间: 2019-10-14 10:17

AnnieLinbaby 发表于 2019-10-14 02:20
. 有一个八条道的操场，直的两条道是100miles，最内圈的两个半圆直径是64miles，每条道比里面的多1mile，问 ...

想一下学校里的跑道就行了啊，八条道的跑道，直边长度一样，差别就在左右两个半圆上，求最外圈和最内圈两个圆的差

作者: zhandoudaodi 时间: 2019-10-14 14:56

AnnieLinbaby 发表于 2019-10-14 02:34
问(3/4-2/9)-2/3和3/4-(2/9-2/3)的差距，我选的3/4less
楼主，怎么感觉
-2/3 - 2/3 -> -4/3 呀

我和你算的一样 -4/3

作者: AnnieLinbaby 时间: 2019-10-14 14:57
【45-补充】
PS：几何题，一个等腰直角三角形组合一个长方形，给了等腰直角三角形的斜边28，有一个角是30°，然后问一个边长?
构筑答案：很简单，看了就能做。我选的14（√6-√2）

楼主，这题答案确定吗？能不能解释一下
麻烦你啦

作者: zhandoudaodi 时间: 2019-10-14 14:58
20，30，x三个数，问算术平均数比极差大吗？1）x小于10；2）x大于5

请问这题是选择D吗，因为条件都可以回答否定，还是要选择E啊，凌乱了谢谢

作者: AnnieLinbaby 时间: 2019-10-14 14:59

vieooo 发表于 2019-10-14 10:17
想一下学校里的跑道就行了啊，八条道的跑道，直边长度一样，差别就在左右两个半圆上，求最外圈和最内圈两 ...

楼主><
16pie这个答案是确定的吗？
我想象了一下还是觉得有点难理解！能告诉我公式吗

作者: Jasper_Zhou 时间: 2019-10-14 16:12
328 miles 的跑道，瑟瑟发抖

作者: Jasper_Zhou 时间: 2019-10-14 16:16

AnnieLinbaby 发表于 2019-10-14 14:59
楼主><
16pie这个答案是确定的吗？
我想象了一下还是觉得有点难理解！能告诉我公式吗 ...

如果每条跑道只比里面条直径多1M的话，应该是8pi （72pi-64pi）才对。感觉像是每条道宽1m.

作者: goldenbabegril 时间: 2019-10-15 01:27
第一题最里面那条64mile的算第一条跑道吗？如果算的话第八条是64+7=71 pi吧，也就是说一共是7*2=14pi，是这样吗。如果最里面一条不算跑道那就是16

作者: 咩咩咩咩咩2333 时间: 2019-10-15 11:29

zhandoudaodi 发表于 2019-10-14 14:58
20，30，x三个数，问算术平均数比极差大吗？1）x小于10；2）x大于5

请问这题是选择D吗，因为条件都可以回 ...

我觉得只有条件一才能回答否吧，如果是条件二的话平均数可能比极差大也可能比极差小呀

作者: Jasper_Zhou 时间: 2019-10-15 14:29

zhandoudaodi 发表于 2019-10-14 14:58
20，30，x三个数，问算术平均数比极差大吗？1）x小于10；2）x大于5

请问这题是选择D吗，因为条件都可以回 ...

我也觉得应该选A，只要x<10，那平均数一定<20，极差一定>20，故极差一定>平均，可以回答 "是否"

作者: 林大牛 时间: 2019-10-16 23:30

Jasper_Zhou 发表于 2019-10-15 14:29
我也觉得应该选A，只要x平均，可以回答 "是否"

同意！

作者: 泼盎胖 时间: 2019-10-18 10:27

vivivi123 发表于 2019-10-13 23:26
数学第四题不应该是4/3 less嘛还有第七题应该选A对嘛

第四题同意！

作者: 泼盎胖 时间: 2019-10-18 10:50
第六题我的理解是条件一平均数肯定不大于极差
条件二可能大可能小
所以只有一能回答题目的问题
不知道对不对

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