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Bunuel
The rate R at which a chemical reaction in a certain industrial process proceeds is a function of time t and is given by \(R = at^3 + bt^2 + c\), where a, b, and c are constants and t > 0. Is there a positive value of t for which R = 0 ?

If we leave all that mambo jumbo out, the question essentially asks whether \(at^3 + bt^2 + c = 0\), has a solution for t, where t > 0.

(1) a > b

If all three constants are positive and t is positive, then \(at^3 + bt^2 + c\) becomes the sum of three positive values and cannot equal 0. So, in this case, no value of positive t would satisfy \(at^3 + bt^2 + c = 0\).

However, in other cases, for example, if a = 0, b = -1, and c = 1, the equation becomes t^2 = 1, which has a positive t satisfying it (t = 1).

Not sufficient.

(2) c > 0.

Similarly, we can deduce the insufficiency of this statement.

(1)+(2) Combining the statements, we still lack sufficient information. Again, we can consider the cases:

If all three constants are positive and t is positive, then \(at^3 + bt^2 + c\) becomes the sum of three positive values and cannot equal 0. So, in this case, no value of positive t would satisfy \(at^3 + bt^2 + c = 0\).

However, in other cases, for example, if a = 0, b = -1, and c = 1, the equation becomes t^2 = 1, which has a positive t satisfying it (t = 1).

Not sufficient.

Answer: E.
­This question confused me a bit as it asks "is there a positive value of t for which R=0".  For statement 1 you already gave an example which proves yes, there is a positive value.  There can be many positive values.  Same with statement 2.

Or does "is there a positive value of t" mean one and only one value?
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turd_fergsn

Bunuel
The rate R at which a chemical reaction in a certain industrial process proceeds is a function of time t and is given by \(R = at^3 + bt^2 + c\), where a, b, and c are constants and t > 0. Is there a positive value of t for which R = 0 ?

If we leave all that mambo jumbo out, the question essentially asks whether \(at^3 + bt^2 + c = 0\), has a solution for t, where t > 0.

(1) a > b

If all three constants are positive and t is positive, then \(at^3 + bt^2 + c\) becomes the sum of three positive values and cannot equal 0. So, in this case, no value of positive t would satisfy \(at^3 + bt^2 + c = 0\).

However, in other cases, for example, if a = 0, b = -1, and c = 1, the equation becomes t^2 = 1, which has a positive t satisfying it (t = 1).

Not sufficient.

(2) c > 0.

Similarly, we can deduce the insufficiency of this statement.

(1)+(2) Combining the statements, we still lack sufficient information. Again, we can consider the cases:

If all three constants are positive and t is positive, then \(at^3 + bt^2 + c\) becomes the sum of three positive values and cannot equal 0. So, in this case, no value of positive t would satisfy \(at^3 + bt^2 + c = 0\).

However, in other cases, for example, if a = 0, b = -1, and c = 1, the equation becomes t^2 = 1, which has a positive t satisfying it (t = 1).

Not sufficient.

Answer: E.
­This question confused me a bit as it asks "is there a positive value of t for which R=0".  For statement 1 you already gave an example which proves yes, there is a positive value.  There can be many positive values.  Same with statement 2.

Or does "is there a positive value of t" mean one and only one value?
­
The question asks whether \(at^3 + bt^2 + c = 0\) has a solution for t, where t > 0. Here, a, b, and c are constants—specific fixed numbers, but we don't know their values. For some combinations of a, b, and c, there will be a solution for t, where t > 0; for others, there will not. Together, statements (1) and (2) specify that a > b and c > 0, narrowing the possible values of these constants. However, as shown above, even then, we can have two different outcomes: if these constants are positive, the answer is NO, and if these constants are, for example, 0, -1, and 1, then the answer is YES.
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Can we construct equations such that the rate of reaction R becomes negative? This probably will not happen in the real world, and we do kind of use real-world common sense.
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Bunuel,

The question does not asks whether we have sufficient information to say FOR SURE that there is a positive value of t for which r =0.


it asks if there if a positive value of t.

if T =1 then there is a positive value for which R=0, therefore statement 1 should be sufficient.
Bunuel
turd_fergsn
When it asks is there a value does that not mean is it possible to find a situation where it can be done?
­
No, it does not mean that. If that were the case, then why would we even need the statements? We could say right away that yes, for some specific values of a, b, and c, there is a positive value of t for which R equals 0. However, that's not what the question means. We are given an expression R = at^3 + bt^2 + c, where a, b, and c represent some specific unknown numbers and two additional pieces of information (a > b and c > 0). The question asks whether we have sufficient information to say for sure that there is a positive value of t for which R equals 0, which, as explained above, we don't.
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In summary, data sufficiency demands certainty, not just possibility.
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Dbrunik
Bunuel,

The question does not asks whether we have sufficient information to say FOR SURE that there is a positive value of t for which r =0.


it asks if there if a positive value of t.

if T =1 then there is a positive value for which R=0, therefore statement 1 should be sufficient.
Bunuel
turd_fergsn
When it asks is there a value does that not mean is it possible to find a situation where it can be done?
­
No, it does not mean that. If that were the case, then why would we even need the statements? We could say right away that yes, for some specific values of a, b, and c, there is a positive value of t for which R equals 0. However, that's not what the question means. We are given an expression R = at^3 + bt^2 + c, where a, b, and c represent some specific unknown numbers and two additional pieces of information (a > b and c > 0). The question asks whether we have sufficient information to say for sure that there is a positive value of t for which R equals 0, which, as explained above, we don't.

Not sure I follow you, but here it is again: a, b, and c in the question are specific, fixed constants; we just don't know their values. The question asks whether the statements provide enough information to conclude for sure whether there is a positive value of t for which at^3 + bt^2 + c = 0. If all three constants are positive and t is positive, the answer is no. However, if a = 0, b = -1, and c = 1, then the answer is yes.
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jm2k5
The rate R at which a chemical reaction in a certain industrial process proceeds is a function of time t and is given by \(R = at^3 + bt^2 + c\), where a, b, and c are constants and t > 0. Is there a positive value of t for which R = 0 ?

(1) a > b
(2) c > 0­




Responding to a pm:

Is there a positive value of t for which R = 0?

i.e. is there a value of t for which \(at^3 + bt^2 + c = 0\)

Constraints: t is positive

Our answer depends on the values of a, b and c. Say if all a, b and c are positive, then since t is positive too, all 3 terms will be positive and their sum can never be 0.
But if at least one of the three constants is negative or all are 0, then we can have values of t for which the expression is 0.

(1) a > b
Doesn't tell us anything about positive/negative. Not sufficient.

(2) c > 0­
c is positive but a and/or b could still be negative to give 0. Not sufficient.

Now take all data together,
Say if both a and b are positive, then no, there is no value of t for which this expression will be 0.
But say if both a and b are negative, then this expression could easily be 0. e.g.\( -2t^3 - 3t^2 + c = 0\) The first and second terms are both negative but c could be large enough to compensate for them and make the equation 0. Then yes, it is possible to have a value for t in which the equation is 0.

Answer (E)
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I just looked at the discriminant:

b^2-4ac

for (1) if a>b there are cases where the discriminant is positive and others where it is negative : not sufficient

for (2) this tells us nothing about b or a so same as (1): not sufficient

Together-- Still not sufficient, let a=1, b=2, and c=3 and it is negative; whereas, a=10, b=11 and c=1 is positive

E
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The question asks if there is a positive value of t for R=0. So by statements 1 and 2 we can say that there indeed exists positive value of t where R=0. So should'nt the answer be D?
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Reetisha
The question asks if there is a positive value of t for R=0. So by statements 1 and 2 we can say that there indeed exists positive value of t where R=0. So should'nt the answer be D?
No. The question asks whether we can determine for sure if such a positive t exists, not whether it’s merely possible. Since a, b, and c are fixed but unknown, and different sign combinations give different outcomes, the answer remains E. Study the full discussion above carefully, it explains this distinction in detail.
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Differentiate the equation - we get 3at^2+2bt = 0
Solving it t( 3at+2b) =0
Since t cannot be zero
t=-2b/3a
now to arrive at where t is positive we need to know the signs of a and b which is not provided in either statement 1 or 2 --> hence E
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