What is the value of a?

Question

(1) a - b + 4 = 0
(2) a = -a

Since "a" = -a, 2a = 0, or a = 0 Am I missing something here? How does 2a = 0? Confused about this one, given that a is an unknown variable. Thanks

zisis · Answer

source?
cannt understand why 2a = 0, but my understanding is that only 0 satisfies a = -a , try plugging positive, negative and fractions in the place of a....

HustleHarder · Answer

Source is the ARCO GMAT/GRE book. I don't understand how -a = 0 To me (intuitively) if a is positive, -a is negative, and if a is negative, -a is positive. Hrumphhhh!

Anonymous · Answer

This question has issues, so don't spend too much time thinking about it. Zero is neither positive nor negative. I seriously doubt you would see a question on the real GMAT about positive zero vs. negative zero. GMAT tends to avoid topics that are ambiguous.

Bunuel · Answer

What is the value of a?

(1) a - b + 4 = 0 --> two variables, one equation, so we can not find uniques numerical value of  a . Not sufficient.

(2) a = -a --> number equals to minus itself, only number to satisfy this is zero, so  a=0 . Sufficient.

Or:  a=-a  --> add  a  to both sides:  a+a=-a+a  -->  2a=0  -->  a=0 .

Answer: B.

You are right "if a is positive, -a is negative, and if a is negative, -a is positive". But also if  a=0 , then  -a=a=0  (0=-0).

Bounce1987 · Answer

1st statement is definitely not sufficient.
2nd statement is sufficient if you add a to both sides. 2a=0 => a=0

ScottTargetTestPrep · Answer

a - b + 4 = 0

This means a = b - 4. However, since we don’t know anything about b, statement one alone is not sufficient to determine the value of a.

Statement Two Alone:

a = -a

2a = 0

a = 0

Statement two alone is sufficient to answer the question.

Answer: B

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What is the value of a?

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