GMAT考试-Testprep数学精解(9)

ica，“ and L stand for ”you can lead a life of leisure.“ Now the first senten

ce translates as W——>S， the second sentence as S——>L， and the conclusion as

W——>L. Combining these symbol statements yields the following diagram：

W——>S

S——>L

Therefore， W——>L

The diagram clearly displays the transitive property.

DeMorgan's Laws

~（A & B） = ~A or ~B

~（A or B） = ~A & ~B

If you have taken a course in logic， you are probably familiar with these fo

rmulas. Their validity is intuitively clear： The conjunction A&B is false wh

en either， or both， of its parts are false. This is precisely what ~A or ~B

says. And the disjunction A or B is false only when both A and B are false，

which is precisely what ~A and ~B says.

You will rarely get an argument whose main structure is based on these rules

——they are too mechanical. Nevertheless， DeMorgan's laws often help simplify

， clarify， or transform parts of an argument. They are also useful with game

s.

Example： （DeMorgan's Law）

It is not the case that either Bill or Jane is going to the party.

This argument can be diagrammed as ~（B or J）， which by the second of DeMorga

n's laws simplifies to （~B and ~J）。 This diagram tells us that neither of th

em is going to the party

A unless B

~B——>A

“A unless B” is a rather complex structure. Though surprisingly we use it wi

th little thought or confusion in our day-to-day speech.

To see that “A unless B” is equivalent to “~B——>A，” consider the following s

ituation：

Biff is at the beach unless it is raining.

Given this statement， we know that if it is not raining， then Biff is at the

beach. Now if we symbolize “Biff is at the beach” as B， and “it is raining”

as R， then the statement can be diagrammed as ~R——>B.

CLASSIFICATION

In Logic II， we studied deductive arguments. However， the bulk of arguments

on the GMAT are inductive. In this section we will classify and study the ma

jor types of inductive arguments.

An argument is deductive if its conclusion necessarily follows from its prem

ises——otherwise it is inductive. In an inductive argument， the author presen

ts the premises as evidence or reasons for the conclusion. The validity of t

he conclusion depends on how compelling the premises are. Unlike deductive a

rguments， the conclusion of an inductive argument is never certain. The trut

h of the conclusion can range from highly likely to highly unlikely. In reas

onable arguments， the conclusion is likely. In fallacious arguments， it is i

mprobable. We will study both reasonable and fallacious arguments.

We will classify the three major types of inductive reasoning——generalizatio

n， analogy， and causal——and their associated fallacies.

Generalization

Generalization and analogy， which we consider in the next section， are the m

ain tools by which we accumulate knowledge and analyze our world. Many peopl

e define generalization as “inductive reasoning.” In colloquial speech， the

phrase “to generalize” carries a negative connotation. To argue by generaliz

ation， however， is neither inherently good nor bad. The relative validity of

a generalization depends on both the context of the argument and the likeli

hood that its conclusion is true. Polling organizations make predictions by

generalizing information from a small sample of the population， which hopefu

lly represents the general population. The soundness of their predictions （a

rguments） depends on how representative the sample is and on its size. Clear

ly， the less comprehensive a conclusion is the more likely it is to be true.

Example