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For non-zero integers a, b, c and d, is ab/cd positive? (1)

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For non-zero integers a, b, c and d, is ab/cd positive? (1) [#permalink] New post 26 Oct 2006, 15:36
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For non-zero integers a, b, c and d, is ab/cd positive?

(1) ad + bc = 0

(2) abcd = -4

I say D.
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 [#permalink] New post 26 Oct 2006, 15:44
(D) as well :)

ab/cd > 0

Stat 1 :
ad + bc = 0
<=> ad = -bc

Since a, b, c and d are not equal to zero, we have
ad/bc = -1

Multiplying or dividing by number gives the same sign to the result, thus

Sign(ad/bc) = Sign(ab/cd) < 0

SUFF

Stat 2 :
Same reasonning as above:

Sign(a*b*c*d) = Sign(ab/cd) < 0

SUFF
  [#permalink] 26 Oct 2006, 15:44
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For non-zero integers a, b, c and d, is ab/cd positive? (1)

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