Friday

August 28, 2015
Posted by **Heather** on Monday, August 4, 2014 at 10:48pm.

The answer choices are:

(A) 8a^2 + 6

(B) 8a^2 + 12

(C) 12a^2 + 6

(D) 18a^2 + 6

(E) 18a^2 + 12

Thank you!

- Math -
**Heather**, Monday, August 4, 2014 at 10:57pmActually, just did it again and got (D). However, it's an SAT question and I need the fastest way to solve it... what I did was plug in values for a and b (3 and 2, respectively) then find c (78) and plug a (3) back into each of the choices to see if c (78) and the answer were equal. Is there a faster way to do it?

- Math -
**Steve**, Monday, August 4, 2014 at 11:17pmStart substituting:

1/3c = 6b^2, so

c = 18b^2

Now, 3b=2a, so

18b^2 = 2(3b)^2 = 2(2a)^2 = 8a^2

I think you left out a +6 somewhere, since there are no such terms shown.

Also, if a=3 and b=2, c=72, not 78, so it's a good sign something has been left out.

So, if D is the answer, the question has been garbled. At any rate, you can see how I did it, so maybe you can apply the logic to the correct problem.

- Math -
**Heather**, Monday, August 4, 2014 at 11:43pmSorry- it was 1/3c = 6b^2 + 2.

I can see your way of substituting, though!