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If a*b = −8, what is the value of a + b? (1) a − b = 6 (2) a^2*b = 32

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If a*b = −8, what is the value of a + b? (1) a − b = 6 (2) a^2*b = 32  [#permalink]

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New post Updated on: 26 Nov 2019, 09:33
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Question Stats:

54% (02:02) correct 46% (01:40) wrong based on 34 sessions

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Originally posted by Bunuel on 25 Nov 2019, 02:13.
Last edited by chetan2u on 26 Nov 2019, 09:33, edited 1 time in total.
Edited statement (2)
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Re: If a*b = −8, what is the value of a + b? (1) a − b = 6 (2) a^2*b = 32  [#permalink]

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New post 25 Nov 2019, 21:24
If \(a*b = −8\), what is the value of \(a + b\)?

(1) \(a − b = 6\)
This will clearly give us two values....a=b+6
So \(a*b=b(b+6)=-8.......b^2+6b+8=0...(b+4)(b+2)=0\)
Two cases
b=-4, a=2...a+b=-4+2=-2
b=-2, a=4...a+b=4-2=2
Insuff

(2) \(a^2*b = 32.......a*(a*b)=32.....a*(-8)=32.....a=-4\)
So b = 2 and a+b=-4+2=-2
Suff

B

However the values of b differs in both statements. It should have been \(a*b^2=32\)
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Re: If a*b = −8, what is the value of a + b? (1) a − b = 6 (2) a^2*b = 32  [#permalink]

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New post 26 Nov 2019, 09:31
Bunuel wrote:
If \(a*b = −8\), what is the value of \(a + b\)?

(1) \(a − b = 6\)

(2) \(a^2*b = 32\)


(1) \(a − b = 6\) insufic

\(a − b = 6…a=6+b…b(6+b)=-8…b^2+6b+8=0…(b+4)(b+2)=0\)
\((b,a)=(-4,2);(-2,4)…(a+b)=(-2;2)\)

(2) \(a^2*b = 32\) sufic

\(a^2*b = 32…a(ab)=32…a=32/(-8)=-4…b=2…a+b=-2\)

Ans (B)

*note: if a^2b=32 then a^2>0 and b>0; but (1) b=-4,-2… so there is something wrong with (2).
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Re: If a*b = −8, what is the value of a + b? (1) a − b = 6 (2) a^2*b = 32   [#permalink] 26 Nov 2019, 09:31
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