SDCAR2010【逻辑入门】（八）Assumptions

sdcar2010

If a question asks for NECESSARY assumption, you can try the negation method. By definition, a necessary assumtion is a claim which is guaranteed to be correct if the original argument is true (thus the modifier NECESSARY). In terms of logic:
If the orginal argument is correct, then the NECESSARY assumption is true.

The countrapositive of the above statement is:
If the NECESSARGY assumption is NOT true (negation), the orginal argument is NOT correct.

So if you negate an answer choice, the original argument falls apart, then that choice is the correct answer for a necessary assumption.

guoqi

Hey, nice to talk to you again,

As for these assumption questions, the general rule is to negate the option to see whether the option can weaken the conclustion. My question is I find lost of questions follow the principle like this:

If  A(  premise), Then B ( Conclusion)

What we need to do is find a necessary assumption C to prove if  without C, B doesn't exit. Right?
But, what is the role of A? , " if, then" sounds like a scieficent premise, But I doubt A is a scieficent premise.... I don't know in such kind of the question, what can A do since C is a necessary assumption. Just want to know that, do you think that I have gone too far beyond the scope?

Thanks

sdcar2010

If A, then B.

A is a premise and B is a conclusion.  A is a sufficient condition for B to occur; B is a necessary condition to happen if A happens. That's it.

sanlingjie

要崩溃了，十道题快错光了！！！！！！啊！！！！！要疯了！！！！

sdcar2010

要崩溃了，十道题快错光了！！！！！！啊！！！！！要疯了！！！！

-- by 会员 sanlingjie (2011/7/2 19:16:38)

I felt your pain. But you have to do it again.

sanlingjie

要崩溃了，十道题快错光了！！！！！！啊！！！！！要疯了！！！！

-- by 会员 sanlingjie (2011/7/2 19:16:38)

I felt your pain. But you have to do it again.

-- by 会员 sdcar2010 (2011/7/2 21:26:13)

刚刚看了你2010的那些逻辑题分析的帖子，结果几乎每一题我都做错了，我真快疯了，差点把自己的脑袋垂爆！我怎么这么笨啊！楼主能不能教教我该怎么办啊！我今天一天都在看你的帖子，每篇都很认真地看，可是一旦做你出的那些练习题，就全完蛋了。我太伤心了！

sdcar2010

No shortcut. Redo the problems and figure out why you chose the wrong answers.

jianaozhonghua

无法区分sufficient和necessary。谁能给我讲讲怎么区别？使用negate的时候应该注意什么。。。

Vicky90

Try this question:

The U.S. census is not perfect: thousands of Americans probably go uncounted. However, the basic statistical portrait of the nation painted by the census is accurate. Certainly some of the poor go uncounted, particularly the homeless; but some of the rich go uncounted as well, because they are often abroad or traveling between one residence and another.

Which of the following is an assumption on which the argument above depends?

(A) Both the rich and the poor have personal and economic reasons to avoid being counted by the census.
(B) All Americans may reasonably be classified as either poor or rich.
(C) The percentage of poor Americans uncounted by the census is close to the percentage of rich Americans uncounted.
(D) The number of homeless Americans is approximately equal to the number of rich Americans.
(E) The primary purpose of the census is to analyze the economic status of the American population.

-- by 会员 sdcar2010 (2011/6/28 20:58:35)

Which is a accurate answer ?  I confused by the option of A and E .

sdcar2010

C) is the answer for the U.S. census question.