Algebra Ch8 Problem Set#6

[tabishsangrar]

If x=(9b-3ab)/((3/a)-(a/3)) , what is x

(1) 9ab/(3+a) = 18/5
(2) b = 1

Shoudn't the answer be (E) here.
Acc. to the explanation, you simplify the main expression to get:

9ab(3-a)/((3-a)(3+a)) and then you cancel (3-a) !!

But can we really cancel (3-a)? We don't know if "a" is not equal to 3 in which case the denominator would be 0.

Please help :( :(

Cheers,
Taz

[RonPurewal]

GMAC's materials state that "all variables and expressions represent real numbers", which means that you can ignore cases that lead to division by zero.

The official problems usually -- but not always -- include conditions that rule out those values anyway, just so the situation is more clear.

So, you should ignore the case of a = 3, because it doesn't give a real number as an answer. But, sure, we should probably edit the problem to add "If a ≠ 3..." to the beginning.

[YashA953]

Since it's not give that a != 3, shouldn't the answer be C? With the information in both the statements, you should be able solve for a and b.

[RonPurewal]

you ignore the case of a = 3, for the reasons already explained in the post directly above yours.