Question: Tricky Cubic Equation

Comment on Tricky Cubic Equation

isn't K's sign going to affect the answer, let say we solved the equation and we got 3 values for k = -11/3, +1 , -1 .. In first k= -11/3 case when we substitute the value of k in both the quantities the quantity B is GREATER, the value of k= +1 , -1 quantity A is greater,
How can we be sure that k value won't effect the choice ???
greenlight-admin's picture

First of all, we need not solve for k. We can find the correct answer without solving the given equation.
That said, if k = -11/3, then Quantity A = 3 + 11(-11/3) = 3 - 121/3 = -112/3
If k = -11/3, then Quantity B = 11(-11/3) - 3 = 121/3 - 3 = -130/3
Quantity A is greater (you suggested that Quantity B is greater)

Yea really tricky. I didn't realized that i don't need k.

By far one of my most favorite videos ever! The "confession" is truly classic. I certainly did fall into the "tricky" mental category. I laughed so hard at first and then cried how easy the question is. Wonderful video.

So I have a quick question , so what if K is not a constant but a negative variable. then subtracting K wont be right . I mean how do I know if a variable in the question is a constant or a variable ?? I Solved it assuming K is a variable and can be negative.
greenlight-admin's picture

Be careful. We can safely ADD or SUBTRACT any value (positive or negative) from both quantities.
However, we must avoid MULTIPLYING or DIVIDING both quantities by a negative value.

More here:


Ah! my bad sir. thank you :)

Haha Nice question! I solved it correctly, but got the answer and second guessed my solution because of the word " tricky" above. Thanks Brent!
greenlight-admin's picture

Ha! Thanks for sharing!

this question blew my mind

Have a question about this video?

Post your question in the Comment section below, and a GRE expert will answer it as fast as humanly possible.

Change Playback Speed

You have the option of watching the videos at various speeds (25% faster, 50% faster, etc). To change the playback speed, click the settings icon on the right side of the video status bar.

Let me Know

Have a suggestion to make the course even better? Email us today!

Free “Question of the Day” emails!