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x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x^-^1 - 5^x?

1) z < 25 and w = 7x 2) x = 4

Note: The above expression is 7^(x-1) -5^x.

I guess statement (1) should be: z<25 and w=7^x.

"x is an integer and x raised to any odd integer is greater than zero" means x=integer>0. Q: is w-z>5(7^{x-1}-5^x)?

(1) z<25 and w=7^x --> is 7^x-z>5(7^{x-1}-5^x)? --> is 7^x-z>5*7^{x-1}-5^{x+1}? --> is 7^x-5*7^{x-1}+5^{x+1}>z? --> as the lowest value of x is 1, then the lowest value of LHS is when x=1: LHS_{min}=7^x-5*7^{x-1}+5^{x+1}=7-5+25=27 --> so the lowest value of LHS is 27, which is more than z as z<25 --> hence 7^x-5*7^{x-1}+5^{x+1}>z is true. Sufficient.

Re: Is w - z greater than 5 times the quantity 7^x-1 - 5^x [#permalink]
20 May 2012, 08:33

Could you please reply with the detailed explanation for the above question. i am confused as to how just Statement I is sufficient to answer the question(A), without knowing the value of X?

Re: Is w - z greater than 5 times the quantity 7^x-1 - 5^x [#permalink]
20 May 2012, 08:52

Hello, let me try and explain.

paraphrasing the questions gives us, is w-z > 5.7^(x-1) - 5^(x+1). Further, x is a positive integer as an odd power of x is greater than 0.

taking statement 1, you have the question as 7^x-z > 5.7^(x-1) - 5^(x+1)

Now, for all cases where x is a positive integer, 7^x would be greater than 5.7^(x-1). Next, 5^(x+1) would always be >= 25 as the least value of x can be 1.

So, 7^x > 5.7^x-1 and z < 5^(x+1). Reversing this inequality and adding both the inequalities, we get:

Re: Is w - z greater than 5 times the quantity 7^x-1 - 5^x [#permalink]
20 May 2012, 09:21

Expert's post

the question is not simple but if you think a bit abstract and test number you can arrive to the solution.

The FIRST important thing, breaking the problem, is to understand tha our X is positive. Infact, if a number raised to power of 3 is >0 that means X itself is positive because the odd powr maintain the original sign of the number, so X must be positive.

At this point using the exponent rules we have w - z > 5*7^x-1 - 5^x+1 ----> 7^x - 24 (from stem x<25) > 5*7^x-1 - 5^x+1

testing number 1 ------> 7 -24 > 1 - 25 ---> - 17 > -24 this is TRUE. also if you test some value < 25 positive but also negative the result is the same.

1) sufficient

x = 4 -------> w - z > 5*7^3 - 5^5..........at this point you know nothing about w and z.

2) insufficient

A is the answer.
_________________

KUDOS is the good manner to help the entire community.

Re: Is w - z greater than 5 times the quantity 7^x-1 - 5^x [#permalink]
03 Jun 2012, 00:27

Bunuel wrote:

carcass wrote:

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x-1 - 5^x?

1 z < 25 and w = 7^x 2 x = 4

Merging similar topics. Please ask if anything remains unclear.

why can't x be zero apart from even ? any odd number raised to the power zero is 1 which is an interger greater than zero. I think x is given as an interger and not only positive integer. if you work with x = 0 , statement 1 is not sufficient ?

Re: Is w - z greater than 5 times the quantity 7^x-1 - 5^x [#permalink]
03 Jun 2012, 01:38

Expert's post

Kaps07 wrote:

Bunuel wrote:

carcass wrote:

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x-1 - 5^x?

1 z < 25 and w = 7^x 2 x = 4

Merging similar topics. Please ask if anything remains unclear.

why can't x be zero apart from even ? any odd number raised to the power zero is 1 which is an interger greater than zero. I think x is given as an interger and not only positive integer. if you work with x = 0 , statement 1 is not sufficient ?

We are told that integer x raised to to any odd integer is greater than zero: x^{odd}>0. Now, if x=0 then x^{odd}=0^{odd}=0 (for odd>0), which violates given condition, so x cannot be zero.

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x^-^1 - 5^x?

1) z < 25 and w = 7x 2) x = 4

Note: The above expression is 7^(x-1) -5^x.

I guess statement (1) should be: z<25 and w=7^x.

"x is an integer and x raised to any odd integer is greater than zero" means x=integer>0. Q: is w-z>5(7^{x-1}-5^x)?

(1) z<25 and w=7^x --> is 7^x-z>5(7^{x-1}-5^x)? --> is 7^x-z>5*7^{x-1}-5^{x+1}? --> is 7^x-5*7^{x-1}+5^{x+1}>z? --> as the lowest value of x is 1, then the lowest value of LHS is when x=1: LHS_{min}=7^x-5*7^{x-1}+5^{x+1}=7-5+25=27 --> so the lowest value of LHS is 27, which is more than z as z<25 --> hence 7^x-5*7^{x-1}+5^{x+1}>z is true. Sufficient.

(2) x = 4. No info about w and z. Not sufficient.

Answer: A.

I don't understand the function of statement z<25. If lowest value of LHS is 27 which is more than z, then it doesn't fulfill the condition given?

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x^-^1 - 5^x?

1) z < 25 and w = 7x 2) x = 4

Note: The above expression is 7^(x-1) -5^x.

I guess statement (1) should be: z<25 and w=7^x.

"x is an integer and x raised to any odd integer is greater than zero" means x=integer>0. Q: is w-z>5(7^{x-1}-5^x)?

(1) z<25 and w=7^x --> is 7^x-z>5(7^{x-1}-5^x)? --> is 7^x-z>5*7^{x-1}-5^{x+1}? --> is 7^x-5*7^{x-1}+5^{x+1}>z? --> as the lowest value of x is 1, then the lowest value of LHS is when x=1: LHS_{min}=7^x-5*7^{x-1}+5^{x+1}=7-5+25=27 --> so the lowest value of LHS is 27, which is more than z as z<25 --> hence 7^x-5*7^{x-1}+5^{x+1}>z is true. Sufficient.

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^x^-^1 - 5^x?

1) z < 25 and w = 7x 2) x = 4

Note: The above expression is 7^(x-1) -5^x.

I guess statement (1) should be: z<25 and w=7^x.

"x is an integer and x raised to any odd integer is greater than zero" means x=integer>0. Q: is w-z>5(7^{x-1}-5^x)?

(1) z<25 and w=7^x --> is 7^x-z>5(7^{x-1}-5^x)? --> is 7^x-z>5*7^{x-1}-5^{x+1}? --> is 7^x-5*7^{x-1}+5^{x+1}>z? --> as the lowest value of x is 1, then the lowest value of LHS is when x=1: LHS_{min}=7^x-5*7^{x-1}+5^{x+1}=7-5+25=27 --> so the lowest value of LHS is 27, which is more than z as z<25 --> hence 7^x-5*7^{x-1}+5^{x+1}>z is true. Sufficient.

(2) x = 4. No info about w and z. Not sufficient.

Answer: A.

Please explain why the lowest value of X is 1.

See the highlighted part: x is an integer and x raised to any odd integer is greater than zero means that x is a positive integer, thus its lowest possible value is 1.

Re: x is an integer and x raised to any odd integer is greater [#permalink]
16 Jun 2013, 20:48

1

This post received KUDOS

Expert's post

ykaiim wrote:

x is an integer and x raised to any odd integer is greater than zero; is w - z greater than 5 times the quantity 7^(x-1)-5^x?

(1) z < 25 and w=7^x (2) x = 4

Responding to a pm:

What is the meaning of: x is an integer and x raised to any odd integer is greater than zero? It is just a convoluted way of saying 'x is a positive integer'. Since x raised to any odd power is positive, x must be positive. x can take values 1, 2, 3, 4 and so on.

Question: Is w - z > 5*7^{x-1}-5^{x+1}

Using statement 1: Is 7^x - 25 > 5*7^{x-1} - 5^{x+1}

Even though we are given that z is less than 25, let's assume it to be 25. In case we can prove that left hand side is greater than right hand side for z = 25, we can prove that left hand side will remain greater than right hand side for any other value of z. If z has a smaller value, say 9, left had side will become even larger (since a smaller number will be subtracted) than the right hand side.

Now note that 7^x will be greater than 5*7^{x - 1} since 7^x can be written as 7*7^{x - 1} (x is a positive integer). Also note that minimum value of x is 1 so 5^{x + 1} will always be equal to or larger than 25 (x is a positive integer).

So comparing the left and right hand side expressions, 7^x is greater than 5*7^{x - 1} and 25 is less than or equal to 5^{x+1}. So left hand side must be greater than the right hand side in all cases when x is a positive integer.