# Incredible frequency of careless mistakes [closed]

Ever since high school, I’ve had a serious problem with math classes. Be it discrete math, algebra, calculus or linear algebra, I seldom have trouble understanding the texts or lectures, but when it comes to executing calculations on homework and exams, I can never do it: I keep making mistakes. It’s been a constant source of frustration, and right now, I’m on the verge of failing linear algebra and second year calculus (which I’ve been avoiding for as long as I could).

I’m currently going through all the exercises and videos on Khan Academy, beginning with one-digit addition, working my way back up to integrals. I was making a lot of mistakes with multiple digit subtraction and multiplication, so initially, I was hopeful that maybe with enough practice in those two things, I’d be good to go again. But it turns out that my mistakes aren’t confined to any one task, and the number of mistakes I make per exercise just accumulates as the tasks become more and more computationally involved:

• Thinking that 7 + 5 = 13
• Neglecting negative signs when transcribing
• Carrying in multiple-digit addition when a digit hasn’t surpassed 10
• Using the wrong exponent laws, log laws, limit laws etc.
• Mixing up trigonometric function properties
• Forgetting to reverse signs when expanding negated expressions of multiple terms

A typical page from my notebook:

I’ve tried everything: going for walks, ensuring that I have good sleep, writing out every step with almost hilarious verbosity, changing rooms, starting with fresh notebooks, reviewing everything I write, trying different notations, heck I even spent a month refining my handwriting and posture for good measure.

I’m certain I spend about twice as much time as other people practicing too. I avoid taking other courses in the same semesters that I take math courses.

As far as personality goes, in general, I’m not exactly OCD, but I’m not careless either. I probably have more patience than most people — a bit too much, as my friends would tell me. I’m sure you can also tell from my adequate grammar, punctuation and spelling that I probably don’t have any kind of learning disability that would prevent me from following symbolic rules. I’ve gotten near 100% in a first order logic course too.

Yet when I do math, I always screw up. It’s been this way for over a decade, maybe more.

I don’t know what I’m doing wrong anymore, and I’m losing hope.

What could I do?

@Rei, I’ve gone through pretty much the same problems as you in school, and also made the same sort of mistakes! And I’ll assume yours isn’t a medical problem. You mentioned a big list of mistakes as being your problem. You probably need to ask yourself first: is there something fundamentally wrong in your understanding, or is it that you just make mistakes when pressed for time, etc.? If it’s the former, get the concept right first, saving the calculations for later. If it’s the latter, it’s a much more common problem that we all get into. I can’t give you a more specific answer in that case without knowing more about when you make these careless mistakes.

If that seems too abstract, here’s some concrete help about your $x=480\times 72$ problem. Don’t do it multiple times in the same way – instead check whether the answer makes sense in different ways! (This is an extremely important step that I’ve seen many people neglect, because they get a correct answer. That’s really not understanding maths.) First off , $x = (480 \times 72) > (y=480 \times 70)$, where $y$ should be very easy to do :$y= 48 \times 7 \times 100 = 33600$. This alone should tell you that , if you got the correct answer in your worksheet, it must be 34560!

Next step: (I assume you know basic algebra) We know that $x = 480 \times 70 + 480 \times 2 = 480 \times 70 + 960$. If that seems too difficult to compute, just take $x \approx 480 \times 70 + 1000 = 34600$. Now, we finally get your answer of $34560$, by subtracting 40!

Ok, suppose you’re still not sure. That’s fine, we’ll do another quick check. It’s easier $z= 500 \times 70$ which is just $5\times 7$ with three zeros to follow, so $z=35000$. And $x$ should be a little less; if you’re interested you can find the difference. Otherwise, you can check your computations by finding which answer you got was closest to $35000$, and discarding the rest.

What have we gained in the process?

• We admitted that certain computations are hard to do by hand.
• Hence, we looked for concepts to simplify those computations, and looked for approximate answers.
• We checked our answers several times, in multiple ways.

Here’re some thumb rules to keep in mind:

• Human beings make errors in a variety of situations, so you’re no exception!
• Since you mentioned errors in multi-digit addition, subtraction , etc. you may want to try those first,ignoring everything beyond.It’s not important that you compute very fast, but it’s very important that you get the algorithms very clearly.
• When you get an answer, just stop and ask, “how can I check this”? Check your answer any way you want – use a calculator, computer, etc.

Here’re some simple exercises to try out; you may want to try finding an approximate answer first, without doing any pen-and-paper calculation:

• $99 \times 99$
• $3.14 \times 2.99$
• $-1 - (-1)$

As an example, I’ll give some hints for the first one. Let $a=99^2$. I find it hard to square 99, so I’ll just square 100 and say $a$ is slightly less than 10000. Next, I’ll maybe use an identity like $a=(100-1)^2$ and zoom in on how much $a$ is less than 10000.
I’ll perform a crude check by stating that $a>90 \times 90 = 8100$. So $a$ lies between $8100$ and $10000$.

Hope I’ve taken the keen edge off your despair!

Does that help?