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设随机变量X和Y的联合分布是正方形G=(X,y):1≤x≤3,1≤y≤3上的均匀分布,试求随机变量U=|X-Y|的概率密度p(u).

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Compare with the short bow, what is the advantage of the long bow

A. Long range.
B. Light weight.
C. Beautiful design.
D. Aiming device.

Many students find the experience of attending university lectures to be a confusing and frustrating experience. The lecturer speaks for one or two hours, perhaps (61) the talk with slides, writing up important information on the blackboard, (62) reading material and giving out (63) The new student sees the other students continuously writing on notebooks and (64) what to write. Very often the student leaves the lecture (65) notes which do not catch the main points and (66) become hard even for the students to understand. Most institutions provide courses which (67) new students to develop the skills they need to be (68) listeners and note-takers. (69) these are unavailable, there are many useful study-skills guides which (70) learners to practice these skills independently. In all cases it is important to (71) the problem before actually starting your studies. It is important to (72) that most students have difficulty in acquiring the language skills (73) in college study. One way of (74) these difficulties is to attend the language and study-skills classes which most institutions provide throughout the (75) year. Another basic strategy is to find a study partner with whom it is possible to identify difficulties, exchange ideas and provide support.

A. without
B. with
C. on
D. except

进行房地产开发项目可行性分析的核心和重点是研究开发项目的经济效益。某房地产开发项目有四个设计方案,其中甲方案净现值FNPV=200万元,投资现值为3000万元,乙方案净现值FNPV=180万元,投资现值为2000万元,丙方案净现值FNPV=150万元,投资现值为3000万元,丁方案净现值FNPV=200万元,投资现值为2000万元。为了对初步确定方案做进一步分析,拟计算内部收益率,选择不同的折现率进行计算,得到如下结果:当折现率i1=10%时,财务净现值FNPV(i1)=200万元;当折现率i2=12%时,财务净现值FNPV(i2)=-100万元。试根据以上条件,回答下列问题。 被初步选定的最佳方案的内部收益率是( )。

A. 10.0%
B. 10.67%
C. 11.33%
D. 12.0%

Passage Two Our brains could be hard-wired to be male or female long before we begin to grow testes(丸)or ovaries(卵巢)in the womb. This discovery might explain why some people feel trapped in a body that’s the wrong sex, and could also lead to tests that reveal the true "brain sex" of babies born with ambiguous genitalia(生殖器). Till now, the orthodoxy among developmental biologists has been that embryos develop ovaries and become female unless a gene called SRY on the Y chromosome is switched on. If this gene is active, it makes testes develop instead. This switch is seen as the key event in determining whether a baby is a girl or a boy. Only after the gonads(性腺) form and flood the body with the appropriate hormones, the theory goes, is the sex of our minds and bodies determined. But in a study of mice, a team at the University of California, Los Angeles, has now found that males and females show differences in the expression of no fewer than 50 genes well before SRY switches on. "It’s the first discovery of genes differentially expressed in the brain, "says Eric Vilain, who led the UCLA team. "They may have an impact on the hard-wired development of the brain in terms of sexual differentiation independent of gonadal induction." Vilain is presenting details of seven of the 50 genes to the annual meeting of the American Society of Human Genetics in Baltimore this week. Three of these genes are dominant in females and four are dominant in males. The next step for Vilain and his team will be to show that the genes in question really do influence brain sexuality—and not just in mice. This is likely to be a much tougher proposition than merely showing there are differences in expression. But if the findings are confirmed, they could one day yield blood tests that allow doctors to establish the brain sex of babies born with genitalia that share features of both sexes. At present doctors and parents have to guess which gender to assign for surgical "correction". The first paragraph tells us that ______.

A. whether we will be male or female is determined by the brain, not the genitalia
B. some people consider themselves to be male but are in fact female physiologically
C. our brain sex is formed long after our testes or ovaries begin to grow
D. some people feel trapped because they have ambiguous genitalia

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