OG倒数的几道题~

Eileenl · 发表于 2010-10-21 12:53:10

229. Right triangle PQR is to be constructed in the xy-plane
so that the right angle is at P and PR is parallel to the
x-axis. The x- and y-coordinates of P, Q, and R are to
be integers that satisfy the inequalities –4 ≤ x ≤ 5 and
6 ≤ y ≤ 16. How many different triangles with these
properties could be constructed?
(A) 110
(B) 1,100
(C) 9,900
(D) 10,000
(E) 12,100
225. A straight pipe 1 yard in length was marked off in
fourths and also in thirds. If the pipe was then cut into
separate pieces at each of these markings, which of
the following gives all the different lengths of the
pieces, in fractions of a yard?
(A)
1/6
and
1/4
only
(B)
1/4
and
1/3
only
(C)
1/6
,
1/4
, and
1/3
(D)
1/12
,
1/6
, and
1/4
(E)
1/12
,
1/6
, and
1/3
213. In an electric circuit, two resistors with resistances x
and y are connected in parallel. In this case, if r is the
combined resistance of these two resistors, then the
reciprocal of r is equal to the sum of the reciprocals of
x and y. What is r in terms of x and y ?
(A) xy
(B) x + y
(C)
1/x + y
(D)
xy/x + y
(E)
x + y/xy
主要是看不明白题，请会的指点指点~谢谢啦

Eileenl · 发表于 2010-10-28 18:46:38

ding

yinverse · 发表于 2010-10-28 19:17:24

最后一题选D，说的是电阻的并联吧~
第二题也选D，问的是把1码平分为4份，同时平分为3份，切开，则一共有多少种长度？可以想象成一个数轴，在0到1间同时标4等分点和3等分点，则依次是0,1/4,1/3,1/2,2/3,3/4,1，算之间的间距有多少种~ 算出来是1/4,1/6,1/12
求问第一题

Eileenl · 发表于 2010-10-28 19:29:39

就是（1/x+1/y）的倒数是不？

yinverse · 发表于 2010-10-28 19:40:04

对~~~
第一题咋算呐，NN快出来~~~~

yinverse · 发表于 2010-10-28 19:51:53

自力更生了找到第一题答案了，选C
我琢磨应该是A(10,2)*A(11,2)=10*9*10*11=9900.之前算错是把A错想成C了，其实应该有顺序问题的，因为题干没说PRQ的方位，所以顶点Q可以在PR上方也可以在PR下方，P也不一定在R左边，还可以在R右边~

OG倒数的几道题~

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