求助 马上要考了~prep数学题还是有几道不会~大家帮忙看看~settled

zyq19911205

我怎么发现prep的数学还挺绕的啊~~gmat数学也没有传说中的那么简单啊~~~数学看来要悲催了~



这题我是直接用等比数列求和算出结果了，有没有更先进的方法啊~
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小女子先跪谢了~~

zyq19911205

恩，直接看成是与面积和厚度都成正比

-- by 会员 energyhunter (2012/8/8 4:45:30)

恩~明白了~~多谢~~~

zyq19911205

for 21:
As cost is proportional to thickness and square of length, we can assume:
c(Cost) = k * t(thickness) * len^2
from (1): k*2^2*0.2 - k*2^2*0.1= 0.4K = 160 , so k = 400,
so cost of 3*0.1 square slab is c= 400*3^3*0.1=360.  Sufficient
(2) is same to (1). Sufficient.
for 27:
from (1), I = 1000(1+r/100)^2 - 1000 = 210, 1+r/100 = (1.21)^(1/2), r is unique. so (1) is sufficient.
for (2), we can only know 1+r/100 >(1.15)^(1/2), replace r with 0.08, 1.08^2 = 1.166 > 1.15 , so r may less then 0.8 but still met (2). (2) is insufficient.

-- by 会员 greengreed (2012/8/8 1:19:08)

明白了~太感谢了~~~

energyhunter

恩，直接看成是与面积和厚度都成正比

greengreed

for 21:
As cost is proportional to thickness and square of length, we can assume:
c(Cost) = k * t(thickness) * len^2
from (1): k*2^2*0.2 - k*2^2*0.1= 0.4K = 160 , so k = 400,
so cost of 3*0.1 square slab is c= 400*3^3*0.1=360.  Sufficient
(2) is same to (1). Sufficient.
for 27:
from (1), I = 1000(1+r/100)^2 - 1000 = 210, 1+r/100 = (1.21)^(1/2), r is unique. so (1) is sufficient.
for (2), we can only know 1+r/100 >(1.15)^(1/2), replace r with 0.08, 1.08^2 = 1.166 > 1.15 , so r may less then 0.8 but still met (2). (2) is insufficient.

eriesun

21同求

zyq19911205

顶一下~

zyq19911205

第三个，2^6,3*2^5,

-- by 会员 jsmart (2012/8/7 9:00:28)

太感谢啦~~上面是我又不会的两道题~~能帮我再看看么~~

zyq19911205

第一题 1/2 -1/4 1/8 -1/16...
容易看出，前两项加起来是1/4，后面偶数项，两两相加奇数项绝对值都大于紧跟的偶数项绝对值，因此这个数列和>1/4
第一项1/2，后面是-1/4 1/8 -1/16... 两两合并，相加得数都<0，且最后多出来一项也是负数，因此与1/2相加得出数列和<1/2

第二题跟一楼说的一样，3^4n=81^n  几个81相乘个位数都是1 3^(4n+2)个位数一定是9，m=1 就可以算余数了

第三题 算最小的先2^6=64，是两位数，然后感觉差不多快接近100了，就挨个带 3*2^5=96，后面就没可能了所以是俩

-- by 会员 烧卖居士 (2012/8/7 11:13:59)

太感谢了~~明白了~~我又做了另一份prep，又有两道题不会，请大侠指教~~


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