Rejecting X Values

Added: 2026-05-25

[00:00:00]Here's how you can get from F9 to A1 for rejecting X values.

[00:00:05]Let's take a look at this solving equation. You have X over X plus three equals to 2X minus four over 3X plus nine. So, when we do cross multiply, you will do X 3X plus nine equals to X plus three 2X minus four. Okay, so you're supposed to expand.

[00:00:27]You will have 3X squared plus >> [music] >> uh 9X equals to 2X squared minus 4X plus 6X minus [music] 12. So, you're supposed to bring everything over to the left-hand side.

[00:00:45]So, 9X This is plus [music] 2X bring over, so minus 2X plus 12 equals to zero. You have X plus 7X plus 12 equals to zero.

[00:00:59]Okay, so you're supposed to factorize this.

[00:01:03]And you will have X plus three X plus four equals to zero. So, X equals to minus three or X equals to four. So, this is a F9 student common mistake. So, what they will do is oh okay, so everything looks good and they will accept both of the answer.

[00:01:25]If you want to get A1, here is what you should do. So, it looks like it is correct. No issue, everything is perfect, but there is one problem. So, over here, you must know that when denominator involved X, denominator involved X, you always need to check any X value do you need to reject. So, in this case, our X plus three cannot be zero because when denominator is zero, it will be math error. So, X cannot be >> [music] >> minus three. So, one value you must reject. So, you also have to test this one because here another denominator that has X. So, in this case, 3x + 9 cannot be zero. X + 3 cannot be zero because I divide by three. X cannot be -3. So, happens to be the same. So, that means you know that only X -3 you have to reject. And over here, you cannot accept the X -3 and you have to say it is reject. So, do you remember how to reject this answer?

[00:02:29]So, for algebraic fraction equation, you always check whether the denominator involved X and if it involved X, you must always check which X value you need to reject. I am Teacher Kenji. I turn F9 students into A1 students. Follow for more or join my classes.

[00:02:47]>> [music]