Is there any simple method to calculate $\sqrt x$ without using logarithm Suppose that we don't know logarithm, then how we would able to calculate $\sqrt x$, where $x$ is a real number? More generally, is there any algorithm to calculate $\sqrt [ n ]{ x } $ without using logarithm? More simple techniques would be nice.
Here is a simple technique used to approximate square roots by Persian author Hassan be al-Hossein:
For example: $\sqrt {78}\approx 8\frac { 14 }{ 17 } $ , where $8$ is the nearest integer root of $78$, $14 = 78 - 8^2$, $17 = 2 \times 8 + 1$.
if $n=2^k$ we can use the method above. 
For example, for $k=2$ Lets calculate $\sqrt [ 4 ]{ 136 } $: $$\sqrt [ 4 ]{ 136 } =\sqrt { \sqrt { 136 }  } \approx \sqrt { 11\frac { 136-{ 11 }^{ 2 } }{ 11\times 2+1 }  } =\sqrt { 11\frac { 15 }{ 23 }  } \\ \sqrt { 11\frac { 15 }{ 23 }  } \approx 3\frac { 11\frac { 15 }{ 23 } -{ 3 }^{ 2 } }{ 3\times 2+1 } =\frac { 544 }{ 161 } =3.38\\$$ The exact result is$$ \sqrt [ 4 ]{ 136 } =3.4149\cdots$$ The method approximates well, but it is working for only $n=2^k$ as I know. 
 A: $$f(x)=\sqrt [ n ]{ x }\Rightarrow f'(x)=\frac {x^{(1/n-1)}}{n}$$
$$f'(x_0)\approx\frac{f(x_0+h)-f(x_0)}{h}$$
$$\Rightarrow f(x_0+h)\approx f'(x_0)h+f(x_0)$$
Suppose you want to  calculate $f(x)=\sqrt [ 3 ]{ x }$ at $x=7 $ then 
take $h=-1$ and $x_0=8$
$$f(7)\approx f'(8)(-1)+f(8)\approx-\frac{1}{12}+2\approx\frac{23}{12}$$
A: The continued fraction method works like this:  Suppose $x = a^2 + b$, where $a = \lfloor \sqrt x \rfloor$. Then 
$$
\begin{align}
x &= \sqrt{a^2 + b}\\
x-a &=  \sqrt{a^2 + b} - a\\
\frac{1}{x-a} &= \frac{1}{\sqrt{a^2 + b} - a}\\
              &= \frac{1}{\sqrt{a^2 + b} - a}\frac{\sqrt{a^2 + b} + a}{\sqrt{a^2 + b} + a}\\
              &= \frac{\sqrt{a^2 + b} + a}{b}\\
              &= \frac{x + a}{b}
\end{align}
$$
Substitute, and get:
$$
\begin{align}
x &= a + (x-a)\\
  &= a + \frac{b}{a+x}\\
  %= a + \frac{b}{2a+\frac{b}{a+x}}\\
x &= a+\cfrac{b}{2a+\cfrac{b}{2a+\cfrac{b}{2a + \dots}}}
\end{align}
$$
Now, this is not a simple continued fraction.  However, if one divides the numerator and denominator of $\frac{b}{2a+x}$ by $b$, then one can eventually get a periodic simple continued fraction, and one approximates by the convergents.  The above expression turns out to be faster.
A: If $x$ is an integer, then you can find the continued fraction expansion of $\sqrt x$ and get very close approximations with no division involved. If you want 6-place accuracy, for instance, continue till you get a convergent with denominator $>1000$.
A: For $y=\sqrt{x}$ there is a simple method:
\begin{align}
y &= 1 &&\text{initialize} \\
 y &=\frac {(\frac{x}{y}+y)}{2} & &\text{repeat until convergence} 
\end{align}
It can be modified for roots of higher orders.
A: You can binary search the answer of the nth root of $x$.
Set $A$ = number you know it's below the nth root and $B$ = one you know is higher then calculate $A+B/2$ if $((A+B)/2)^n \neq x$ then set $B = (A+B)/2$ and repeat (you can always choose $A = 0$ and $B = x$ or $A = 0$ and $B = 1$ if x < 1).
A: To get $\sqrt[n]{a}$ solve the equation $x^n = a$, e.g. with Newton's method.
A: There is an old-fashioned digit-by-digit method that I learned when I was at school. The theory of it is explained here with a base 10 example here, and many old arithmetic books give more practical details for actually carrying out the calculations in a sensible manner.
I have a very old arithmetic textbook which does something similar for cube roots, but it gets more tedious, and I have never seen anything for 5th roots.
A: The method by hand, is to use the method of long division with changing divisor.  You can do this without a calculator, and works for all numbers.
The trick relies on $(x+d)^2 = x^2 + 2xd + d^2$.  One has $x$ as a multiple of 10, and $x^2$ is the bit already subtracted, so you subtract at the next instance $(2x+d).d$.  The answer is doubled, and an underscore after it, eg for 78, we have 16._ * _ is less than (78-64).  
This is a worked example, with commentry, of the finding of the square root by long division with changing divisor.  The new digit is set in brackets here: normally one might write an underscore.
Note digit-paring to assist in finding the estimate of the next place.  The place directly after the six is a zero, which means you go (0)(x) at that point.
The pairing of digits must be so that the radix or decimal point falls between a pair.  So 1 44 .  is paired with the odd digit at the front.
            8 .  8   3   1   7   6  0  9 
          ----------------
         ) 78   00
 (8)       64              8^2 is the largest under 78
           --
  16(8)    14   00         16_ is 2*8, find _ that 16_ * _ is less than 1400
           13   44         _ = 8
           -------
                56  00      difference 56, bring down a pair of zeros.
  176(3)        52  89      176 is twice 88,  176x * x gives x=3
                -------
                 3  11  00       difference, bring down two zeros.
  1 76 6(1)      1  76  61       2 would be too big   
                -----------
                 1  34  39  00     difference, bring down two more zeros
  17 66 2(7)     1  23  62  89     We see that 7 comes to be the next digit.
                 --------------
                    10  76  11  00
  1 76 65 4(6)      10  59  72  44    Here we begin to round by discarding places.
                   ---------------    That is we we just multiply d * x.
                        16  57  56
   17 66 54 ..                         too big, return a 0
    1 76 65 [4]         15  89  49     9

A: To compute $\sqrt{5}$, for example, you can find a solution to $x^2 - 5 = 0$ using Newton's method.
A: You can use Taylor expansion of function $(1+x)^{\frac{1}{n}}$
$$(1+x)^{\frac{1}{n}} = \sum_{k=0}^{\infty}x^k(-1)^k {{1/n}\choose{k}} = \sum_{k=0}^{\infty} \frac{x^k(-1)^k}{k!}(1/n)(1/n-1)(1/n-2)\ldots(1/n-(k-1)) = \sum_{k=0}^{\infty} \frac{x^k}{k!n^k}(n-1)(2n-1)\ldots((k-1)n-1)$$
It is simple to calculate the sum. For each $k$ you can use the values of sum for $k-1$
A: Calculate $\sqrt [ 4 ]{ 136 }$ using interpololation.
$3^4 = 81$
$4^4 = 256$
$256 - 81 = 175$
$136-81 = 55$
$\sqrt [ 4 ]{ 136 } \approx  3 + \frac{55}{175}$
We started with rough estimations, but now we're going to tenths:
$34^4 = 1336336$
$35^4 = 1500625$
$1500625 - 1336336 = 164289$
$1360000 - 1336336 = 23664$
$\sqrt [ 4 ]{ 136 } \approx  3.4 + \frac{23664}{164289} \times 10^{-1} \approx 3.4144$
If we want the error to be no more than $10^{-3}$ then at this point we can check
$\tag 1 3414^4 = 135848255916816$
$\tag 2 3415^4 = 136007491950625$
and confidently state that
$$\tag 3 3.414 \lt \sqrt [ 4 ]{ 136 } \lt 3.415$$
But what if $\text{(1)}$ and $\text{(2)}$ had not straddled $136$? That would be more than upsetting!
A: $$\sqrt[3]{x}=1+\frac{x-1}{3}-\frac{2(x-1)^2}{9(2!)}+\sum_{n=1}^\infty (-1)^{n-1} \frac{(2+3n)(2+3(n-1))(x-1) ^n)}{3n(n!)}$$
A: Every positive number $x$ can be always written as the sum of two other numbers (or the difference).
Say that one of the two is a perfect square $n^2$, then we can always say that
$$x = n^2 + q$$
Where $q$ is the obvious remainder. Since we can also adopt the difference convention, it's better to write 
$$x = n^2 \pm q$$
When to choose the plus or the minus? Well the rule is that the smaller is $q$, the better.
After that, we can use the following approximation:
$$\sqrt{x} = \sqrt{n^2 \pm q} \approx n \pm \frac{q}{2n}$$
Example
Let's calculate $\sqrt{40}$. Either we choose $40 = 36 + 4$ or $40 = 49 - 9$. The first one is better of course, hence
$$\sqrt{40} = \sqrt{36 + 4} \approx 6 + \frac{4}{12} = 6 + \frac{1}{3} = 6.\bar 3$$
Notice that the real value is
$$\sqrt{40} = 6.324(...)$$
