Question

How to Find Square Root

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Answer

Answer: Use one of several methods — recognize perfect squares or use prime factorization for exact integers, the long-division (manual square-root) method for digit-by-digit results, or Newton’s (Babylonian) method for fast decimal approximations. Below I explain each with examples and tips.

Quick overview

  • If the number is a perfect square (1,4,9,16,25,…), its square root is the integer whose square gives the number.
  • For integers, prime-factorization can give an exact integer/root-simplified form.
  • For non-perfect or decimal numbers, use Newton’s method (fast convergence) or the long-division method (digit-by-digit).
  • Calculators and programming languages provide built-in sqrt functions.

1) Perfect squares and prime factorization

Example: Find $\sqrt{3600}$.

  • Prime factorize $3600 = 2^4 \times 3^2 \times 5^2$.
  • Take one factor of each pair: $\sqrt{3600} = 2^2 \times 3 \times 5 = 4 \times 3 \times 5 = 60$.

Rule: Pair up identical prime factors; each pair contributes one factor to the square root.

2) Long-division (manual digit) method

Use this when you want exact decimal digits without a calculator.

Example: $\sqrt{1521}$.

  1. Group digits from the decimal point in pairs: 15 | 21.
  2. Largest square ≤ 15 is $3^2=9$. First digit 3. Subtract 15−9=6. Bring down 21 → 621.
  3. Double current root (3 → 6). Find digit $d$ so that $(60 + d)d \le 621$. Try $d=9$: $(60+9)9=69\times9=621$. So $d=9$.
  4. Root is 39 and remainder 0. So $\sqrt{1521}=39$.

(The method works similarly for decimals by adding pairs of zeros after the decimal point.)

3) Newton’s (Babylonian) method — fast for decimals

To approximate $\sqrt{S}$, iterate
$$x_{n+1}=\frac{x_n + S/x_n}{2}.$$
Example: approximate $\sqrt{20}$.

  • Choose initial $x_0=5$ (since $5^2=25$).
  • $x_1=(5 + 20/5)/2=(5+4)/2=4.5$.
  • $x_2=(4.5 + 20/4.5)/2\approx(4.5+4.444444)/2\approx4.472222$.
  • $x_3\approx4.472136$ → converges quickly to $\sqrt{20}\approx4.472136$.

Newton’s method typically doubles correct digits each iteration once close.

4) Estimation + refinement

  • Find nearest perfect squares. E.g., $16<20<25$, so $\sqrt{20}$ is between 4 and 5.
  • Interpolate or use one Newton step from an estimate to refine.

5) Using a calculator or programming

  • In calculators use sqrt or x^(1/2).
  • In many languages: Python: import math; math.sqrt(20) or 20**0.5.

6) Special notes

  • For negative numbers, square roots are complex: $\sqrt{-a}=i\sqrt{a}$ for $a>0$.
  • Property: $\sqrt{ab}=\sqrt{a}\sqrt{b}$ for nonnegative $a,b$; $\sqrt{a^2}=|a|$.

If you want, tell me a specific number (integer or decimal) and I’ll show one of these methods step-by-step for that number.