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

energyhunter

恩，直接看成是与面积和厚度都成正比

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~~~

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~~~