math35qa

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course Mth 151

3/18/12 3:48PM

016. Translating Arguments 

 

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Question: `q001. There are 6 questions in this set.

 

Is the following argument valid? 'If it rains, the grass will get wet. If the grass gets wet, we'll be able to smell the wet grass. It rained yesterday. Therefore yesterday we were able to smell the wet grass.'

 

 

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Your solution:

The following argument is valid, because it states the grass will get wet if it rains, and if it rains you’ll be able to smell the grass. Then it says yesterday it rained and you could smell the grass, which is what is supposed to happen if it rains.

 

 

confidence rating #$&*: 3

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Given Solution:

This argument certainly seems valid. We say what will happen if rains, and what will happen is that happens. Then we say that it rains, so the whole chain of happenings, rained then wet grass then smell, should follow.

 

 

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Self-critique (if necessary): OK

 

 

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Self-critique Rating: OK

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Question: `q002. Is the following argument valid: 'If it snows, the roads will be slippery. If the roads are slippery they'll be safer to drive on. Yesterday it snowed. Therefore yesterday the roads were safer to drive on.'

 

 

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Your solution:

The statement is valid because it says what will happen if it snows and what happens when that happens. Then it says it snowed and the roads were slippery and safer to drive on, so all the things happened like they were stated to happen.

 

 

confidence rating #$&*:

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Given Solution:

The validity of an argument has nothing to do with whether the statements in that argument are true or not. All we are allowed to do is assume that the statements are indeed true, and see if the conclusions of the argument therefore hold. In this case, we might well question the statement 'if the roads are slippery they'll be safer to drive on', which certainly seems untrue. However that has nothing to do with the validity of the argument itself. We can later choose to reject the conclusion because it is based on a faulty assumption, but we cannot say that the argument is invalid because of a faulty assumption.

 

This argument tells us that something will happen if it snows, and then tells us what we can conclude from that. It then tells us that it snows, and everything follows logically along a transitive chain, starting from from the first thing.

 

STUDENT COMMENT:  so it does not matter that the roads are not safer when they're slippery, what matters is that the statement said they are when snows and snowed yesterday therefore the roads were safer yesterday

 

INSTRUCTOR RESPONSE:  Right. The statements don't have to be true for the argument to be valid. Of course, if the statements aren't true then even though the argument is valid the conclusion might not be true. The old saying is 'garbage in, garbage out'. If you put 'garbage' (i.e., false statements) into a logical argument, that argument can indeed result in 'garbage' (i.e., a false statement as the logical conclusion).

 

STUDENT COMMENT:

 

According to the statement it is true, but I might question this about driving on slippery roads.

 

INSTRUCTOR RESPONSE:

 

That assumption is deliberately absurd, to help make a clear distinction between correct assumptions and correct logic.

 

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Self-critique (if necessary):

 

 

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Self-critique Rating:

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Question: `q003. Is the following argument valid: 'Today it will rain or it will snow. Today it didn't rain. Therefore today it snowed.'

 

 

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Your solution:

The argument is valid because it says it will either rain or snow, so one or the other has to happen. It said it didn't rain so it will snow, which is corrected because one of them has to happen.

 

 

confidence rating #$&*: 3

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Given Solution:

If we accept the fact that it will do one thing or another, then at least one of those things must happen. If it is known that if one of those things fails to happen, then, the other must. Therefore this argument is valid.

 

 

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Self-critique (if necessary): OK

 

 

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Self-critique Rating: OK

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Question: `q004. Is the following argument valid: 'If it doesn't rain we'll have a picnic. We don't have a picnic. Therefore it rained.'

 

 

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Your solution:

This argument is valid because it says if it doesn't rain we'll have a picnic, then it says we didn't have a picnic so it rained, which would be true because the only reason they would have a picnic is if it rained.

 

 

confidence rating #$&*: 3

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Given Solution:

In this argument where told the something must happen as a result of a certain condition. That thing is not happen, so the condition cannot have been satisfied. The condition was that it doesn't rain; since this condition cannot have been satisfied that it must have rained. The argument is valid.

 

 

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Self-critique (if necessary): OK

 

 

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Self-critique Rating: OK

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Question: `q005. We can symbolize the following argument: 'If it rains, the grass gets wet. If the grass gets wet, we'll be able to smell the wet grass. It rained yesterday. Therefore yesterday we were able to smell the wet grass.' Let p stand for 'It rains', q for 'the grass gets wet' and r for 'we can smell the wet grass'. Then the first sentence forms a compound statement which we symbolize as p -> q. Symbolize the remaining statements in the argument.

 

 

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Your solution:

“If it rains, the grass gets wet” can be symbolized as p → q, “if the grass gets wet, we'll be able to smell it” can be symbolized as q → r, and It rained yesterday can be symbolized by p. The symbolized statement would be [(p → q) ^ (q → r) ^ p] → r, because all three parts leads to the grass being wet.

 

 

confidence rating #$&*: 2

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Given Solution:

The argument gives three conditions, 'If it rains, the grass gets wet. If the grass gets wet, we'll be able to smell the wet grass. It rained yesterday.', which are symbolized p -> q, q -> r and p. It says that under these three conditions, the statement r, 'we can smell the wet grass', must be true. Therefore the argument can be symbolized by the complex statement [ (p -> q) ^ (q -> r) ^ p] -> r.

 

STUDENT COMMENT:  becuase the statment is valid r will be on the outside of the parenthesis

INSTRUCTOR RESPONSE:  It doesn't matter whether the statement is valid or not. 

The premises go into the parentheses or brackets, the conclusion follows the -> sign.

The form of the argument is

[premises] -> conclusion,

where the premises inside the brackets are joined by conjunctions.

 

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Self-critique (if necessary): OK

 

 

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Self-critique Rating: OK

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Question: `q006. The preceding argument was symbolized as [ (p -> q) ^ (q -> r) ^ p] -> r. Determine whether this statement is true for p, q, r truth values F F T.

 

 

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Your solution:

The statement is true because p → q is true, q → r is false, and p is false, which makes [(p -> q) ^ (q -> r) ^ p] false, and the for the statement to be false [(p -> q) ^ (q -> r) ^ p] would have to be false and r to be true.

 

 

confidence rating #$&*: 3

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Given Solution:

For these truth values p -> q is true since p is false (recall that the only way p -> q can be false is for p to be true and q to be false), q -> r is false since q is false, and p itself is false, therefore [ (p -> q) ^ (q -> r) ^ p] is false. This makes [ (p -> q) ^ (q -> r) ^ p] -> r true, since the statement can only be false if [ (p -> q) ^ (q -> r) ^ p] is true while r is false.

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Self-critique (if necessary):

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Self-critique rating:

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Question: `q006. The preceding argument was symbolized as [ (p -> q) ^ (q -> r) ^ p] -> r. Determine whether this statement is true for p, q, r truth values F F T.

 

 

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Your solution:

The statement is true because p → q is true, q → r is false, and p is false, which makes [(p -> q) ^ (q -> r) ^ p] false, and the for the statement to be false [(p -> q) ^ (q -> r) ^ p] would have to be false and r to be true.

 

 

confidence rating #$&*: 3

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Given Solution:

For these truth values p -> q is true since p is false (recall that the only way p -> q can be false is for p to be true and q to be false), q -> r is false since q is false, and p itself is false, therefore [ (p -> q) ^ (q -> r) ^ p] is false. This makes [ (p -> q) ^ (q -> r) ^ p] -> r true, since the statement can only be false if [ (p -> q) ^ (q -> r) ^ p] is true while r is false.

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Self-critique (if necessary):

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Self-critique rating:

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Question: `q006. The preceding argument was symbolized as [ (p -> q) ^ (q -> r) ^ p] -> r. Determine whether this statement is true for p, q, r truth values F F T.

 

 

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Your solution:

The statement is true because p → q is true, q → r is false, and p is false, which makes [(p -> q) ^ (q -> r) ^ p] false, and the for the statement to be false [(p -> q) ^ (q -> r) ^ p] would have to be false and r to be true.

 

 

confidence rating #$&*: 3

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Given Solution:

For these truth values p -> q is true since p is false (recall that the only way p -> q can be false is for p to be true and q to be false), q -> r is false since q is false, and p itself is false, therefore [ (p -> q) ^ (q -> r) ^ p] is false. This makes [ (p -> q) ^ (q -> r) ^ p] -> r true, since the statement can only be false if [ (p -> q) ^ (q -> r) ^ p] is true while r is false.

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Self-critique (if necessary):

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Self-critique rating:

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