Question: Functions f and g

Comment on Functions f and g

Thank you very much for the tutorial, it was very helpful. Could you please help me understand when questions like these end? To clarify, in the beginning I thought the question was done when we discovered that K = -5
gmat-admin's picture

That's a common trick the GMAT test-takers like to pull. We THINK the goal is to find the value of k, but we ACTUALLY need to find the value of f(k+1)

Hi Brent, as you mentioned we ACTUALLY need to find the value of f(k+1), which is -4 but then why is the need to substitue into f(x) = 3x + 1 again? Thanks Brent.
gmat-admin's picture

Be careful. The value of k+1 is not the same as the value of f(k+1).

Once we learn that k = -5, our goal is to find the value of f(k+1)
So, when we replace k with -5, we get: f(k + 1) = f((-5) + 1) = f(-4)

So, we still need to find the value of f(-4)
Since f(x) = 3x + 1, we know that f(-4) = 3(-4) + 1, which equals 11.

Does that help?

Thanks Brent for great explanation. As is f(-4) rather than -4 itself, therefore it's a sign that it still need substituting back into f(x) to get an integer on it own in this type of function questoin? Is my understanding correct? Thanks Brent
gmat-admin's picture

That's correct.

Hi. I want more practice with questions of this type and difficulty. Can you direct me? I typed in function notation practice on google. But they aren't of this difficulty. They are easier.
gmat-admin's picture

We have links to functions questions in the Reinforcement Activities box here:

Hi Brent,

Just a quick question. Why can't I solve for k by equating f(k+1) = g[(k+1)/2] and then solve it? Theoretically, it should be the same right?
gmat-admin's picture

That approach will kind of work, but you will be forced to perform some VERY TRICKY mental maneuvering during the solution.

Try it and you'll see.


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