How to calculate the square or cubic root of a number on a simple calculator?

I assure you powers are not beyond you conceptually. The computation of fractional powers is burdensome, which is why logarithms, slide rules, and calculators were invented. But calculators have been invented so forget the computation and understand the concepts.

We start with powers that are natural numbers defined as 0, 1, 2, 3, and so on. And, to avoid having to deal with special cases, we talk about the powers of a number greater than zero called the base. We do not restrict the kind of number that is the base except to say it is greater than zero. Let’s call the base b and the power (also called the exponent) p. So b to the p power is written out as [imath]b^p[/imath] or [imath]b[/imath]^[imath]p[/imath].

Remember that initially we are restricting the exponents to whole numbers starting with zero.

The formal definition then is

[math]\text {If } p = 0, \text { then } b^p = 1, \text { but}\\ \text {if } p > 0, \text { then } b^p = b * b^{(p - 1)}.[/math]
So, for example,

[math]3^0 = 1;\\ 3^1 = 3 * 1 = 3;\\ 3^2 = 3 * 3 = 9;\\ 3^3 = 3 * 9 = 27 = 3 * 3 *3;\\ 3^4 = 3 * 27 = 81 = 3 * 3 * 3 * 3;\\ 3^5 = 3 * 81 = 243 = 3 * 3 * 3 * 3 * 3;\\ 3^6 = 3 * 243 = 729; \text { and so on forever.}[/math]
Now remember that the only restriction we placed on b is that it is greater than zero.

So for another example,

[math]\left ( \dfrac{1}{2} \right )^0 = 1;\\ \left ( \dfrac{1}{2} \right )^1 = \dfrac{1}{2};\\ \left ( \dfrac{1}{2} \right )^2 = \dfrac{1}{2} * \dfrac{1}{2} = \dfrac{1}{4};\\ \left ( \dfrac{1}{2} \right )^3 = \dfrac{1}{2} * \dfrac{1}{2} * \dfrac{1}{2} = \dfrac{1}{2} * \dfrac{1}{4} = \dfrac{1}{8}, \text { and so on.}[/math]
So, using the definitions rather than the calculator, what is [imath]0.4^4[/imath]?

Next up, we go for the laws of powers, but are you on board up to here?
 
I'm probably going to need to buy a large set of math "curriculum" books, that go all the way from beginner to advance.

A very bad idea, IMHO. Sorry that I don't know of any good short books suiting your needs, but if you cannot find one (and even if you do) stick to online and, especially, interactive modes: either find a tutor or further exploit this and other online forums.

I spent an obscene amount of time in my youth chewing through thick books, but that time would be orders of magnitudes shorter if I had someone to ask questions or just talk about the topics of interest. I've also noticed the difference between thick and well-written thin books.

Another advice: don't be afraid to make mistakes and ask silly questions. In my long engineering career the best professionals I've met almost uniformly had the same quality: ability to say "I don't know" and "I've made a mistake".
 
So please calculate π * √2 - using addition.
Actually, this is very easy to do with addition, so long as you're willing to accept a certain degree of accuracy and a lot of working out:
π * √2 = 3.14 * 1.41
314 + 314 + 314 + 314 ... (done 141 times) = 44274
Then put the decimal back = ~4.4274 (which is 99.6% accuracy, correct answer for π * √2 = 4.4428)

So I was right all along. Multiplication IS a series of addition. Whether or not it is perfectly accurate, or quick to solve, is really irrelevant.

Here is a quicker and more accurate method:
Multiply Decimals.PNG
 
Last edited:
Incorrect

π * √2 ~ 4.442883

That is what I get from Excel. Whom should I believe - you or Bill Gates?
This proves absolutely nothing! First of all, your answer is also an approximation, just as mine was. Secondly, do you know how Excel or the CPU in your computer calculates the answer? Most likely the computer is doing a series of additions, or a series of efficient multiplications. Even if it was a series of multiplications, it could still theoretically be done with addition, just less efficiently.

Subhotosh, please show me a more efficient and more accurate way to calculate π * √2, without "addition", on paper, to 6 decimals like you quoted was "correct".
 
Last edited:
This proves absolutely nothing! First of all, your answer is also an approximation, just as mine was. Secondly, do you know how Excel or the CPU in your computer calculates the answer? Most likely the computer is doing a series of additions, or a series of efficient multiplications. Even if it was a series of multiplications, it could still theoretically be done with addition, just less efficiently.

Subhotosh, please show me a more efficient and more accurate way to calculate π * √2, without "addition", on paper, to 6 decimals like you quoted was "correct".
This proves absolutely nothing!
This only proves that you need to review the consequence of "=" sign in Mathematics.
do you know how Excel or the CPU in your computer calculates the answer?
I do - I am an engineer. All those answers are "approximately to" and not 'equal to'
to 6 decimals like you quoted was "correct".
If you notice, I had used ~ and not =. Language of Mathematics is accurate and precise (bulls-eye). However, we engineers and physical scientists have to solve real world problems. Thus we settle for "approximate" value.

do you know how Excel or the CPU in your computer calculates the answer
Yes --- I know - I have taken courses in "computer engineering" and "numerical analysis".

show me a more efficient and more accurate way to calculate π * √2
The question was to find "exact" (not more accurate or more efficient) answer - with "=" sign and not "~" sign. Just like you (or I) cannot write the exact perimeter of a circle with 2" diameters (using decimal numerals).

So to come back to your final question and the answer is \(\displaystyle \pi * \sqrt{2}\) cannot be calculated "exactly" (that is mathematics demands when we put = sign) by addition. We cannot even write \(\displaystyle \pi \) exactly with our known numerical base system. These are very good questions. These will be "answered" if you take a series of courses in "Foundations of Mathematics".
 
That's a wonderful way to weasel your way out of this. Pitty you didn't bother to mention that I was "incorrect" due to the symbol that I used, and instead made a comment about Excel and trusting Bill Gates.

I made two posts above your last post, the first proves how to "estimate" π * √2 using a series of addition. I wasn't able to edit the = to ~, but I'm letting you know now what I meant. Clearly, it IS possible to "estimate" π * √2 using addition. So again, I don't understand what the purpose of this unnecessarily confusing goose chase was. It may be tedious to do multiplication using only addition, but you cannot deny the fact that multiplication does fundamentally boil down to addition.

As someone who is a "Super Moderator" of a math forum, I would have expected you to be more helpful, and not deliberately confusing.
 
Last edited:
That's a wonderful way to weasel your way out of this. Pitty you didn't bother to mention that I was "incorrect" due to the symbol that I used, and instead made a comment about Excel and trusting Bill Gates.

I made two posts above your last post, the first proves how to "estimate" π * √2 using a series of addition. I wasn't able to edit the = to ~, but I'm letting you know now what I meant. Clearly, it IS possible to "estimate" π * √2 using addition. So again, I don't understand what the purpose of this unnecessarily confusing goose chase was. It may be tedious to do multiplication using only addition, but you cannot deny the fact that multiplication does fundamentally boil down to addition.
Look. I do not fundamentally disagree with you, provided you limit your consideration to rational numbers, but your analysis breaks down with irrational numbers. You do not get the correct answer with irrational numbers by repeated addition. You get an approximation.

Try showing that your answer works with rational numbers if you define multiplication of two rational numbers as repeated addition plus division.

Give [imath]\dfrac{3}{17} * \dfrac{4}{19}[/imath].
 
But I never said anything about wanting a "correct answer" that stretches out to infinite decimals. I thought that was common sense / assumed. Everything in math needs to be converted to a decimal/number form at some point to be useful in real-world applications.
 
Last edited:
But I never said anything about wanting a "correct answer" that stretches out to infinite decimals. I thought that was common sense / assumed. Everything in math needs to be converted to a decimal/number form at some point to be useful in real-world applications.
You have a hang-up about decimal representation. 1/3 cannot be represented in a finite number of decimals, yet one third times 18 is 6. Why get an approximation to 6 through numerous additions?

The problem with approximation is the potential for build-up error. In many practical applications, there may be thousands of calculations. Each approximation introduces some potential error. After thousands of approximations, that error may become material. In a course on numerical methods, you learn how to minimize such errors and compute their maximum potential size.
Of course, you can estimate

[math]\sqrt{2} * \pi = \left ( \sum_{k = 1}^{141} 314 \right ) \div 10000.[/math]
That means add up 314 141 times and divide the result by 10000. How do you compute the error? (Of course, you can use a calculator, but then why bother with manual calculation at all?)

If we are talking real world applications, aids to manual calculation have been used for hundreds of years and are now enormously powerful. Nowadays, manual calculation has little practical use in advanced technical fields.

If we are talking about concepts, then multiplication as repeated addition makes no conceptual sense with respect to irrational numbers.
 
I still disagree with what you're saying. If you can approximate π * √2 with a finite repeated sum, then I don't see why, conceptually, you couldn't do the same for an infinite repeated sum. I don't see how you can say it makes "no conceptual sense" just because there are infinite decimal places. I would agree with you, if you said "no practical sense".

Does this also make no conceptual sense?
π + π + π ... + π = π*
√2 + √2 + √2 ... + √2 = √2*, which is a smaller infinity than above
 
(see above)

So I can imagine, conceptually, you should be able to do:
((π*∞) * (√2*∞))/∞²

Again, I don't see why you couldn't at least "express" using math, the idea of infinite sums. You could create a math definition for finite sums of length x, and just extend x to infinity. I see the infinity symbol used in calculus all the time.
 
Last edited:
(see above)

So I can imagine, conceptually, you should be able to do:
((π*∞) * (√2*∞))/∞²

Again, I don't see why you couldn't at least "express" using math, the idea of infinite sums. You could create a math definition for finite sums of length x, and just extend x to infinity. I see the infinity symbol used in calculus all the time.
I am not going to argue with you more about this. I have already said that, so long as we are dealing with the rational numbers, I agree with you: you can treat multiplication as repeated addition. I do not enjoy real analysis where things like infinite sums are studied, and I am not going to plunge into studying the one-point compactification of the real numbers where infinity is treated as a number with its own arithmetic. Perhaps through real analysis, non-standard analysis, or one-point compactification, you can develop a rigorous definition of multiplication as an infinite sum.

What I do know is that the moment you start dealing with infinity, our intuition, which was developed from working with finite numbers, can lead us far astray.
 
I guess I will need to learn a lot more math to really understand this. I'm really just guessing here.

Anyway, I appreciate everyone's effort to help, and this thread can now be closed.
 
Top