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Intern
Joined: 07 Oct 2009
Posts: 33
Followers: 0
Kudos [?]:
3
[0], given: 1
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If S and T are non-zero numbers and 1/S+1/T=S+T , which of the following is necessarily true?
(A) ST=1
(B) S+T=1
(C) 1/S=T
(D) S/T=1
(E) none of the above
Please help me understand right answer.I marked A, but it's wrong.
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Tuck Thread Master
Joined: 20 Aug 2009
Posts: 314
Location: Tbilisi, Georgia
Schools: Stanford (in), Tuck (WL), Wharton (ding), Cornell (in)
Followers: 7
Kudos [?]:
68
[0], given: 69
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1/S+1/T=\frac{S+T}{ST}=S+T
S+T=(S+T)*ST
Well so far everything should be pretty obvious
Here are 2 possibilities: either (S+T)=0 OR ST=1. But you cant say that necessarily ST=1, it is one of possible solutions
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Senior Manager
Joined: 30 Aug 2009
Posts: 296
Location: India
Concentration: General Management
Followers: 2
Kudos [?]:
65
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1/S +1/T = (T+S)/TS = S + T => S + T = ST(S + T)
So we have ST = 1 and S+T =0. We also get 1/S =T
now taking the options one by one ---
A) ST=1 ----- possible
(B) S+T=1-----not possible
(C) 1/S=T-----possible
(D) S/T=1---need to check
(E) none of the above
now if S =3 and T = -3 then ST is not equal to 1 so option A and D are out
1/S is not equal to T and S/T is also not equal to 1 so option C and B are also out
so looks like E -none of the above
OA pls
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Intern
Joined: 07 Oct 2009
Posts: 33
Followers: 0
Kudos [?]:
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Thanks Shalva & kp1811.
OA is E.
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