For a finite sequence of non zero numbers, the number of variations in

Hi Bunuel,
Why is -4*1, -4*2 not considered?? You are only taking 1*-3, -3*2 only consecutive terms? Would you please clearify it?

Because 1 and -4 are NOT consecutive terms in the sequence.

I have answered correctly, but my pairs were: (2, -3) (-4,5) (5,-6). My question is, Bunuel, why do we consider (1-3) as pair while (5;-6) not?
Thanks.

Please read the question and the thread carefully. This question is answered here: for-a-finite-sequence-of-non-zero-numbers-the-number-of-127949.html#p1107497

Again, we are told that "the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence ..." 1 and -3 are consecutive terms in the sequence while 5 and -6 are not.

Thank you Bunuel, i got it, i did not realised that 1, -3, 2, 5, -4, -6 is a given finite sequence, for some reason i understood it as a set. Although now i see that if it were a set then the answer would be 0, since there are no pair with negative signs in a normal consequtive sequence.

So the only thing different about this question is that people might re-arrange the sequence and that's what you are not supposed to do?

People might do a lot of things. The point is to read the stem carefully.

Ok it took me like 5 reads to understand what the question is about. I understood Bunuel's explanation (straight forward) but didn't get that GMAT declared a fancy way of saying the product of each pair of integers... I wonder how many of these does it take to drop you off your seat!

The more natural understanding of the number of variations in sign is the number of times a term in the sequence has the opposite sign of its previous term. Because when the sign changes and a term and its preceding term have opposite signs, their product is necessarily negative, so the definition given is functionally equivalent. Understanding that helped me confirm that I understood what was meant by "number of variations in sign."

Keys to this problem: (1) Have in the front of your mind that the product of a positive and negative number is negative, whereas the product of two numbers of the same sign is positive, and (2) understand what number sequences are.

Hi kop,

The GMAT Quant section usually includes at least one "symbolism" question that will either "make up" a math symbol and ask you to perform a calculation with it OR make up a math phrase/concept and ask you to use the concept to answer a question.

These questions are essentially about following instructions.

Here, we're asked to take the PRODUCT of TWO CONSECUTIVE terms. If the product is NEGATIVE, then we have a "variation." So, given the included sequence of numbers, how many "variations" are there? Thankfully the work isn't difficult, but you would need to work through every pair of consecutive terms (and you would find 3 "variations").

These types of questions can sometimes take a little time to solve, but are some of the easiest "math" questions on the exam.

GMAT assassins aren't born, they're made,
Rich

It seems to me that the answer is written explicitly in the question since the question says "the number of variations in sign is defined as the number of paires etc now what does it mean the number of variations? Isn't that the 3 negative signs attached the to the number?
I don't quiet get what they mean the number of variations in sign

Hi Dreams25,

The GMAT Quant section usually includes at least one "symbolism" question that will either "make up" a math symbol and ask you to perform a calculation with it OR make up a math phrase/concept and ask you to use the concept to answer a question.

These questions are essentially about following instructions.

Here, we're asked to take the PRODUCT of TWO CONSECUTIVE terms. If the product is NEGATIVE, then we have a "variation." So, given the included sequence of numbers, how many "variations" are there? Thankfully the work isn't difficult....

1, -3, 2, 5, -4, -6

(1)(-3) = -3 this is a negative produce, so we have 1 'variation'
(-3)(2) = -6 another 'variation'
(2)(5) = 10 NOT a variation (since the product is positive)
(5)(-4) = -20 another 'variation'
(-4)(-6) = 24 NOT a variation

Total variations = 3

Final Answer:

These types of questions can sometimes take a little time to solve, but they are some of the easiest "math" questions on the exam.

GMAT assassins aren't born, they're made,
Rich

Thank you for your response, but I still don't get why do you call 1•-3= -3 a variation. It's a variation compared to what? I can maybe understand a variation in sign when u take 2 negative numbers and multiply them u get a positive but here the negative -3 was negative already before the multiplication so what is the change that we refer as a variation?
Thanks.

Hi Dreams25,

In this prompt, we have to follow the specific instructions that we were given:

"...the number of VARIATIONS in the sign is defined as the NUMBER of PAIRS of CONSECUTIVE terms of the sequence for which the PRODUCT of the two consecutive terms is NEGATIVE."

This defines what a 'variation' is (in this question); you just have to focus on this instruction, and apply it to the given sequence of numbers, to get the correct answer.

GMAT assassins aren't born, they're made,
Rich

(1) Tabulating the problem to see it more clearly.

For a finite sequence of non zero numbers,
the number of variations in sign is defined as the number of pairs of consecutive terms of the sequence for which the product of the two consecutive terms is negative.

What is the number of variations in sign for the sequence 1, -3, 2, 5, -4, -6 ?


(2) Converting to a more compact version.

For a finite sequence of non zero numbers,
V = number of pairs of consecutive terms in {1, -3, 2, 5, -4, -6} when product of the two consecutive terms is negative.

What is V=?

The effort is spent in figuring out what V is. Do not get confused with consecutive terms and consecutive integers. Clearly the author of the problem wants you to confuse the concept of consecutive integers and terms, however, resist the temptation.

In Set S = {a, b, c}, the terms a and b are consecutive terms, as well as b and c. However, a and c are not consecutive terms.

As such, consecutive terms for {1, -3, 2, 5, -4, -6} are:
{1, -3, 2, 5, -4, -6} = 1* -3 = -3 (negative)
{1, -3, 2, 5, -4, -6} = -3 * 2 = -6 (negative)
{1, -3, 2, 5, -4, -6} = 2* 5 = 10 (positive)
{1, -3, 2, 5, -4, -6} = 5 * -4 = -20 (negative)
{1, -3, 2, 5, -4, -6} = -4 * -6 = 24 (positive)

Note that you do not even need to multiply the numbers, however, you need to realize what happens when you multiply a negative times a positive or vice versa.

As a result of the analysis above, you can conclude that you would have three (3) negative pairs.

1, -3, 2, 5, -4, -6

1*-3 = -1
-3*2 = -6
2*5 = 10
5*-4 = -20
-4*-6 = 24

3 negative terms . Variation is 3

We are given the following sequence of numbers: 1, -3, 2, 5, -4, -6.

Every time a pair of consecutive terms product a negative product we have a “variation in sign”. We must determine how many variations in sign are in the sequence.

1 x (-3) = -3, so this is a variation in sign

(-3) x 2 = -6, so this is a variation in sign

5 x (-4) = -20, so this is a variation in sign

Thus, there is a total of 3 variations in sign.

Answer: C