今天小编给大家介绍下在GRE数学中，应该如何比较两个数的大小。一般可以用作差法，作商法，代数法以及列表达式等方法。
　　一、作差法
　　作差法是指两数相减，将相减之后的结果与0比较大小就行，例如x-y>0，就可以判断x>y。
　　3
　　Quantity A：(3 x)/3 Quantity B：(5 x)/x
　　A. Quantity A is greater.
　　B. Quantity B is greater.
　　C. The two quantities are equal.
　　D. The relationship cannot be determined from the information given.
　　【解析】两数做差(3 x)/3-(5 x)/x=x/3-5/x=(x^2-15)/3x，因为30，但是分子x^2-15可能的符号无法确定，所以(x^2-15)/3x与0无法比较大小，所以选D。
　　二、作商法
　　作商法是指两数相除，将相除之后的结果与1比较大小就行，例如x/y>1，就可以判断x>y>0或者x
　　The average(arithmetic mean)height for a group of m men is 70 inches and the average(arithmetic mean)height for a group of f woman is 64 inches.When the groups are combined ,the average height is 66 inches.
　　Quantity A：m Quantity B：f
　　A. Quantity A is greater.
　　B. Quantity B is greater.
　　C. The two quantities are equal.
　　D. The relationship cannot be determined from the information given.
　　【解析】利用十字交叉法可以将两数做商，m/f=(66-64)/(70-66)=1/2 ，选b。
　　三、代数法
　　代数法即带入不同的数字，一般适用于无法比较大小的情况，通过列举不同的数字，发现两数的大小无法判断。
　　n is a positive integer.
　　n is not divisible by 4.
　　n is not divisible by 5.
　　Quantity A：The remainder when n is divided by 4 Quantity B：The remainder when n is divided by 5
　　A. Quantity A is greater.
　　B. Quantity B is greater.
　　C. The two quantities are equal.
　　D. The relationship cannot be determined from the information given.
　　【解析】举例：6除以4余2，除以5余1，2>1即QA>QB；9除以4余1，除以5余4，1 ，所以选d。
　　|x|≥1
　　|y| 　　Quantity A：X/Y
　　Quantity B：Y/X
　　A Quantity A is greater.
　　B Quantity B is greater.
　　C The two quantities are equal.
　　D The relationship cannot be determined from the information given.
　　【解析】x≤-1或者x≥1 -1/2
　　举例：x=1,y=1/4，那么x/y=4，y/x=1/4，QA>QB
　　x=1,y=-1/4，那么x/y=-4，y/x=-1/4，QA
　　无法比较x/y和y/x，所以选D。
　　四、列表达式
　　列表达式是指把题干描述的信息用表达式列出来，或者将几个未知数都用同一个未知数来表示，从而比较大小。
　　Yesterday, Carl had 40 percent more CDs than Karen had. Today, Carl gave 20 percent of his CDs to Karen.
　　Quantity A：Number of CDs that Carl now has
　　Quantity B：Number of CDs that Karen now has
　　A.Quantity A is greater.
　　B.Quantity B is greater.
　　C.The two quantities are equal.
　　D.The relationship cannot be determined from the information given.
　　【解析】假设Carl原来有CDs的数量是C，Karen原来有CDs的数量是K。那么C=1.4K
　　QA：C(1-20%)=0.8*1.4K=1.12K
　　QB：K 20%C=K 0.2*1.4K=1.28K
　　所以QA ，选b。
　　以上就是今天小编给大家介绍的关于GRE数学比大小题型的解题思路，希望对大家有帮助。祝大家早日出分！