Your Number is Up

Something for the weekend.

Let’s start by saying x equals 2.

x = 2

Now multiply both sides with x.

x * x = 2 * x
x2 = 2x

Now subtract 4 from both sides.

x2 – 4 = 2x – 4


(x + 2)(x – 2) = 2(x – 2)

Ah. (x – 2) is a common factor. Eliminate it – capisce?

(x + 2)(x – 2) = 2(x – 2)
x + 2 = 2

Now that means the value of x is…

x + 2 = 2
x = 2 – 2
x = 0

Wait a minute. Isn’t x supposed to be 2??

OK. There is a flaw somewhere.

Figure it out. It’s easy, really. I promise. 🙂

ADDITIONAL NOTE: This is actually from the old KBSM SPM Form 5 syllabus! It was buried in the back of my old Maths text book, and not many students dared venture that far. (I don’t have that text book now, of course, but it’s one of the few things I remember from it…)

And yes, I got an A1 in SPM Maths. Back in the days when getting an ‘A’ meant something…

EXTRA: Just to make sure I WASN’T foisting a ‘ball-busting-brain-buster’ on you nice folks, I tried this question out on my colleague.

He figured out the flaw in 10 seconds. Granted, he holds a Masters in Computer Science, but still….. this IS SPM level, folks.

I was gonna reveal the answer eventually, but now I’m thinking, I wonder how long it’ll take before someone actually sees the light and realises what’s wrong.

So instead of the answer, I give you a CLUE!



16 thoughts on “Your Number is Up”

  1. huh?? *crack brain until near burst*

    i’m never good at maths/figure. (from std. 5 to form 5, never passed my maths/accounts subjects!.) i’m going to give up on this… even before really trying. headache.

  2. Actually you can’t solve it that way. It’s a quadratic equation so you have to make it equal to 0. i.e.

    x2 – 4 = 2x – 4
    x2-2x = 0
    x (x-2) = 0

    x-2 = 0 and x= 0
    x = 2


  3. Goddammit typekey. I meant to say “needs html breaks and things…”

    Anyway, the whole point is just to say is that you have to make everything equal to 0. Yes?

  4. poptart: That wasn’t my question, actually. The point is to point out the flaw in the calculation and why the answer came out to 0 when it is supposed to be 2.

  5. ahhh… the flaw is when you factorise. because that would give you common factor (x-2). If x=2, then your common factor would be 0. 0 divided by 0 is a non-answer. the answer therefore cannot be x=0, it’s inaccurate.

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