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# If S and T are non-zero numbers and

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Director
Status: Done with formalities.. and back..
Joined: 15 Sep 2012
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Location: India
Concentration: Strategy, General Management
Schools: Olin - Wash U - Class of 2015
WE: Information Technology (Computer Software)
Re: If S and T are non-zero numbers and [#permalink]

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11 Feb 2013, 08:59
Sachin9 wrote:
Vips0000 wrote:
Sachin9 wrote:
I get it Vippss. But in all other cases, the generalization will hold good right?

Rules are good, generalizations are not
Enjoy and practice kudos :-p

why on earth do u need kudos u already done with ur gmat. .

Retake bro.. Coming soon
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Lets Kudos!!!
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Re: If S and T are non-zero numbers and [#permalink]

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11 Feb 2013, 09:39
you hav a 720 and you gonna re-take

Good luck mate
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hope is a good thing, maybe the best of things. And no good thing ever dies.

Who says you need a 700 ?Check this out : http://gmatclub.com/forum/who-says-you-need-a-149706.html#p1201595

My GMAT Journey : http://gmatclub.com/forum/end-of-my-gmat-journey-149328.html#p1197992

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Re: If S and T are non-zero numbers and [#permalink]

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18 Mar 2013, 11:06
1
KUDOS
Bunuel wrote:
If S and T are non-zero numbers and $$\frac{1}{S} + \frac{1}{T} = S + T$$, which of the following must be true?

A. $$ST = 1$$
B. $$S + T = 1$$
C. $$\frac{1}{S} = T$$
D. $$\frac{S}{T} = 1$$
E. None of the above

OE:

$$\frac{1}{S} + \frac{1}{T} = S + T$$ --> $$\frac{T+S}{ST}=S+T$$ --> cross-multiply: $$S+T=(S+T)*ST$$ --> $$(S+T)(ST-1)=0$$ --> either $$S+T=0$$ or $$ST=1$$. So, if $$S+T=0$$ is true then none of the options must be true.

Hello Bunuel,

How did u get S+T=(S+T)*ST --> (S+T)(ST-1)=0 ??

What did i miss in he below equation?? how come you got (S+T)(ST-1)=0 ?? Ca you please explain
1/S+1/T = S+T
(S+T) = (S+T) (ST)
divide both side by (S+T) we get
1=1(ST)
therefore ST=1

Thank you.

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Re: If S and T are non-zero numbers and [#permalink]

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18 Mar 2013, 11:12
kuttingchai wrote:
Bunuel wrote:
If S and T are non-zero numbers and $$\frac{1}{S} + \frac{1}{T} = S + T$$, which of the following must be true?

A. $$ST = 1$$
B. $$S + T = 1$$
C. $$\frac{1}{S} = T$$
D. $$\frac{S}{T} = 1$$
E. None of the above

OE:

$$\frac{1}{S} + \frac{1}{T} = S + T$$ --> $$\frac{T+S}{ST}=S+T$$ --> cross-multiply: $$S+T=(S+T)*ST$$ --> $$(S+T)(ST-1)=0$$ --> either $$S+T=0$$ or $$ST=1$$. So, if $$S+T=0$$ is true then none of the options must be true.

Hello Bunuel,

How did u get S+T=(S+T)*ST --> (S+T)(ST-1)=0 ??

What did i miss in he below equation?? how come you got (S+T)(ST-1)=0 ?? Ca you please explain
1/S+1/T = S+T
(S+T) = (S+T) (ST)
divide both side by (S+T) we get
1=1(ST)
therefore ST=1

Thank you.

Check here: if-s-and-t-are-non-zero-numbers-and-141887.html#p1140729

Hope it helps.
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Re: If S and T are non-zero numbers and [#permalink]

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18 Mar 2013, 11:14
kuttingchai wrote:
Bunuel wrote:
If S and T are non-zero numbers and $$\frac{1}{S} + \frac{1}{T} = S + T$$, which of the following must be true?

A. $$ST = 1$$
B. $$S + T = 1$$
C. $$\frac{1}{S} = T$$
D. $$\frac{S}{T} = 1$$
E. None of the above

OE:

$$\frac{1}{S} + \frac{1}{T} = S + T$$ --> $$\frac{T+S}{ST}=S+T$$ --> cross-multiply: $$S+T=(S+T)*ST$$ --> $$(S+T)(ST-1)=0$$ --> either $$S+T=0$$ or $$ST=1$$. So, if $$S+T=0$$ is true then none of the options must be true.

Hello Bunuel,

How did u get S+T=(S+T)*ST --> (S+T)(ST-1)=0 ??

What did i miss in he below equation?? how come you got (S+T)(ST-1)=0 ?? Ca you please explain
1/S+1/T = S+T
(S+T) = (S+T) (ST)
divide both side by (S+T) we get
1=1(ST)
therefore ST=1

Thank you.

I think I got the answer

from your previous post I got "
Never reduce equation by variable (or expression with variable), if you are not certain that variable (or expression with variable) doesn't equal to zero. We can not divide by zero.

So, if you divide (reduce) s+t = (s+t)st by (s+t), you assume, with no ground for it, that (s+t) does not equal to zero thus exclude a possible solution (notice that both st=1 AND (s+t)=0 satisfy the equation).
"

therefore
1/S+1/T = S+T
(S+T) = (S+T) (ST)
0= (S+T) (ST) - (S+T)
0 = (S+T) (ST-1)

(S+T)=0 or ST = 1

Thank you

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Re: If S and T are non-zero numbers and [#permalink]

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12 Jul 2014, 17:12
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Re: If S and T are non-zero numbers and [#permalink]

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26 Dec 2015, 12:34
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If S and T are non-zero numbers and [#permalink]

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27 Dec 2015, 08:46
1
KUDOS
Bunuel wrote:
If S and T are non-zero numbers and $$\frac{1}{S} + \frac{1}{T} = S + T$$, which of the following must be true?

A. $$ST = 1$$
B. $$S + T = 1$$
C. $$\frac{1}{S} = T$$
D. $$\frac{S}{T} = 1$$
E. None of the above

OE:

$$\frac{1}{S} + \frac{1}{T} = S + T$$ --> $$\frac{T+S}{ST}=S+T$$ --> cross-multiply: $$S+T=(S+T)*ST$$ --> $$(S+T)(ST-1)=0$$ --> either $$S+T=0$$ or $$ST=1$$. So, if $$S+T=0$$ is true then none of the options must be true.

Bunuel,

Not able to understand the highlighted part. Kindly explain in detail...

Thx,
Arun

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Re: If S and T are non-zero numbers and [#permalink]

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27 Dec 2015, 08:51
2
KUDOS
Expert's post
ArunpriyanJ wrote:
Bunuel wrote:
If S and T are non-zero numbers and $$\frac{1}{S} + \frac{1}{T} = S + T$$, which of the following must be true?

A. $$ST = 1$$
B. $$S + T = 1$$
C. $$\frac{1}{S} = T$$
D. $$\frac{S}{T} = 1$$
E. None of the above

OE:

$$\frac{1}{S} + \frac{1}{T} = S + T$$ --> $$\frac{T+S}{ST}=S+T$$ --> cross-multiply: $$S+T=(S+T)*ST$$ --> $$(S+T)(ST-1)=0$$ --> either $$S+T=0$$ or $$ST=1$$. So, if $$S+T=0$$ is true then none of the options must be true.

Bunuel,

Not able to understand the highlighted part. Kindly explain in detail...

Thx,
Arun

$$S+T=(S+T)*ST$$

$$(S+T)*ST-(S+T)=0$$

Factor s+t: $$(S+T)(ST-1)=0$$

Hope it's clear.
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Re: If S and T are non-zero numbers and [#permalink]

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07 Sep 2016, 07:06
also the OA can be crosschecked by plugging value of S=1 and T=-1
in this case 1/S+1/T= S+T
but $$ST\neq{-1}$$, $$1/S\neq{T}$$ since S+T=0 here.

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Re: If S and T are non-zero numbers and   [#permalink] 07 Sep 2016, 07:06

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