If f(x) = 12 - x^2/2 and f(2k) = 7k, what is one possible value for k? Answers.
A) 2	B) 3	C) 4	D) 6	E) 7
First of all, see thisGMAT blog postand check the related lesson linked below for some background on function notation.   We can plug anything in for x and get a result. You can find f(1), for example, by plugging in 1 where x is, and you would get 12 - 1/2 = 11.5. Or we could find f(2), which would be  12 - 4/2= 
<gadget id="calculator"> 12 - 4/2</gadget>
<output>10</output>
10.   So the notation f(2k) means that we are going to plug a 2k in for x everywhere in the formula for f(x). That would be:   f(2k)= 12 - (2k)^2/2 = 12 - 2k^2.   Remember that we have to square both the 2 and the k, to get 4k2. Now, this expression, the output, we will set equal to 2k.   12 - 2k^2 = 2k --&gt; k = -3 or k = 7.   All the answers are positive, so we choose k = 2.   Answer = 7
<result>E</result>
If f(x) = 12 - x^2/2 and f(2k) = 7k, what is one possible value for k?