B Determine a fractional square root without calculator

AI Thread Summary
To determine the fractional square root of 100/99 without a calculator, the discussion explores using the Binomial theorem for quick approximations. The initial approach simplifies the expression to (1 + 1/99)^(1/2) and (1 - 0.01)^(-1/2), focusing on the first two terms of the binomial expansion. This leads to an approximation of 1.005, correcting earlier estimates. The conversation emphasizes the importance of using small values of epsilon for accurate approximations in the binomial expansion. Overall, the method provides a practical solution for estimating square roots under time constraints.
vcsharp2003
Messages
913
Reaction score
179
TL;DR Summary
Solving a numerical problem without a calculator
I have to solve a certain numerical problem without using calculator and furthermore, there is a time limit for solving this problem.

The answer I have got so far is ## \sqrt{\frac{100}{99}}##

I know I can reduce the numerator to 10 but then I am stuck with square root of denominator which is not a perfect square.

Question: How would I determine in shortest possible time the above square root without any calculator or tables?
 
Mathematics news on Phys.org
jedishrfu said:
There are several choices outlined here:

https://en.wikipedia.org/wiki/Methods_of_computing_square_roots
Is it possible to use Binomial theorem to do a quick approximation of the fractional square root? This might be better than finding square root of denominator since then I will need to do two approximations, first for denominator square root and then find decimal equivalent for fraction. Binomial theorem might involve a single operation.
 
vcsharp2003 said:
Is it possible to use Binomial theorem to do a quick approximation of the fractional square root?
How could that not be possible?

PS note that ##(\frac{100}{99})^{1/2} = (\frac{99}{100})^{-1/2}##
 
  • Like
Likes docnet and vcsharp2003
PeroK said:
How could that not be possible?
That might be easier. I will try expanding the binomial expression on RHS in equation below and maybe take only the first two terms.

$$\sqrt{\frac{100}{99}} = {(1 +\frac{1}{99})}^{\frac{1}{2}}$$
 
vcsharp2003 said:
That might be easier. I will try expanding the binomial expression below and maybe take only the first two terms.

$${(1 +\frac{1}{99})}^{\frac{1}{2}}$$
Okay, but see above for a useful idea.
 
  • Like
Likes docnet
PeroK said:
Okay, but see above for a useful idea.
I saw your last post. Once I get ##(\frac{99}{100})^{-1/2}##, then how would I proceed?
 
vcsharp2003 said:
I saw your last post. Once I get ##(\frac{99}{100})^{-1/2}##, then how would I proceed?
Use the binomial theorem. This time you have ##(1 - 0.01)^{-1/2}##
 
  • Like
Likes docnet and vcsharp2003
Of course, you could always approximate ##\frac 1 {99} \approx 0.01##, but inverting the fraction feels neater to me.
 
  • Like
Likes docnet and vcsharp2003
  • #10
PeroK said:
Of course, you could always approximate ##\frac 1 {99} \approx 0.01##, but inverting the fraction feels neater to me.
Yes, that's more easier than what I suggested since getting integer powers of .01 is very easy as opposed to getting integer powers of ##\frac{1}{99}##. After taking only first two terms of binomial expansion I get an approximation of 1.005.

$$\sqrt{\frac{100}{99}} = (1 - 0.01)^{-1/2} \approx 1.005$$

First two terms of binomial expression are obtained as below.
$$ (1 - 0.01)^{-1/2} = 1^{-\frac{1}{2}} + (-\frac{1}{2}) 1^ {(-\frac{1}{2} - 1)} (-.01)^1 + \frac{(-\frac{1}{2}) (-\frac{1}{2} -1)}{(1)(2)} 1^ {(-\frac{1}{2} - 2)} (-.01)^2 + ...$$

Taking the first two terms in binomial expression since the terms starting from third term are going to very small ( even smaller than .0001) we get ##1+.005 = 1.005## which seems correct. My initial approximation of .995 was not correct.
 
Last edited:
  • #11
vcsharp2003 said:
Yes, that's more easier than what I suggested since getting integer powers of .01 is very easy as opposed to getting integer powers of 199. After taking only first two terms of binomial expansion I get an approximation of 0.995.

10099=(1−0.01)−1/2≈.995
That can't be right. The answer must be greater than ##1##.

In general $$(1 \pm \epsilon)^{1/2} \approx 1 \pm \frac 1 2 \epsilon$$ and $$(1 \pm \epsilon)^{-1/2} \approx 1 \mp \frac 1 2 \epsilon$$
 
  • Like
Likes docnet
  • #12
PeroK said:
That can't be right. The answer must be greater than ##1##.

In general $$(1 \pm \epsilon)^{1/2} \approx 1 \pm \frac 1 2 \epsilon$$ and $$(1 \pm \epsilon)^{-1/2} \approx 1 \mp \frac 1 2 \epsilon$$
You are correct. I have edited my last post to reflect this. Thanks.
 
  • #13
PeroK said:
That can't be right. The answer must be greater than ##1##.

In general $$(1 \pm \epsilon)^{1/2} \approx 1 \pm \frac 1 2 \epsilon$$ and $$(1 \pm \epsilon)^{-1/2} \approx 1 \mp \frac 1 2 \epsilon$$
Your formulas are awesome. I can memorize these formulas for quicker calculations and not even bother to use Binomial expansion. Thanks.
 
  • #14
vcsharp2003 said:
Yes, that's more easier than what I suggested since getting integer powers of .01 is very easy as opposed to getting integer powers of ##\frac{1}{99}##. After taking only first two terms of binomial expansion I get an approximation of 0.995.

$$\sqrt{\frac{100}{99}} = (1 - 0.01)^{-1/2} \approx .995$$

First two terms of binomial expression are obtained as below.
$$ (1 - 0.01)^{-1/2} = 1^{-\frac{1}{2}} + (-\frac{1}{2}) 1^ {(-\frac{1}{2} - 1)} (-.01)^1 + \frac{(-\frac{1}{2}) (-\frac{1}{2} -1)}{(1)(2)} 1^ {(-\frac{1}{2} - 2)} (-.01)^2 + ...$$

Taking the first two terms in binomial expression since the terms starting from third term are going to very small ( even smaller than .0001) we get ##1+.005 = 1.005## which seems correct.
The usual binomial expansion has a ##1## in the first position. I would always take the factor out first. E.g.
$$(a + \epsilon)^{1/2} = a^{1/2}(1 + \frac{\epsilon}{a})^{1/2}$$ In any case, if you already have ##1## here then: $$(1 + \epsilon)^{1/2} = 1 + \frac 1 2 \epsilon + \frac{1}{2!}(\frac 1 2)(-\frac 1 2)\epsilon^2 + \frac{1}{3!}(\frac 1 2)(-\frac 1 2)(-\frac 3 2)\epsilon^3 \dots$$ Also, to avoid missing minus signs, I would tend to do: $$(1 - \epsilon)^{1/2} = (1 + (-\epsilon))^{1/2} = 1 + \frac 1 2 (-\epsilon) + \frac{1}{2!}(\frac 1 2)(-\frac 1 2)(-\epsilon)^2 + \frac{1}{3!}(\frac 1 2)(-\frac 1 2)(-\frac 3 2)(-\epsilon)^3 \dots$$
 
  • Like
Likes docnet and vcsharp2003
  • #15
PeroK said:
That can't be right. The answer must be greater than ##1##.

In general $$(1 \pm \epsilon)^{1/2} \approx 1 \pm \frac 1 2 \epsilon$$ and $$(1 \pm \epsilon)^{-1/2} \approx 1 \mp \frac 1 2 \epsilon$$
I think these approximation formulas are only valid if ##\epsilon## is very small i.e. as close to 0 as possible on the real number line. Is that correct?
 
  • #16
vcsharp2003 said:
I think these approximation formulas are only valid if ##\epsilon## is very small i.e. as close to 0 as possible on the real number line. Is that correct?
Yes, that's why I used ##\epsilon##. The series converges for any ##-1 < \epsilon < 1##, but it's only useful as an approximation for small ##\epsilon##.
 
  • Like
Likes docnet and vcsharp2003
Back
Top