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How does the Euclidean algorithm iterate step-by-step through consecutive divisions with remainder?

The Euclidean algorithm iterates by repeatedly applying the division with remainder. In each step, the previous divisor becomes the new dividend, and the previous remainder becomes the new divisor. This continues until the remainder is 0, at which point the last non-zero remainder is the greatest common divisor.

Conditions

  • Integers aa and bb satisfy a≥b>0a \ge b > 0.
  • Each step uses the division algorithm to find a quotient and a remainder.

Reasoning, step by step

  1. Start with a=q1b+r1a = q_1b + r_1.
  2. Set the new pair to (b,r1)(b, r_1) and compute b=q2r1+r2b = q_2r_1 + r_2.
  3. Continue with r1=q3r2+r3r_1 = q_3r_2 + r_3, and so on.
  4. Stop when a remainder rn+1=0r_{n+1} = 0.
  5. The greatest common divisor is the last non-zero remainder rnr_n.

Example

The video lists the chain of equations: a=q1b+r1a=q_1b+r_1, b=q2r1+r2b=q_2r_1+r_2, r1=q3r2+r3r_1=q_3r_2+r_3, ..., rn−1=qn+1rn+0r_{n-1}=q_{n+1}r_n+0, showing how the parameters are substituted in each round.

Common misconceptions

  • Believing that the algorithm stops when the quotient is 0.
  • Confusing the last non-zero remainder with the final zero remainder.

Watch the explanation

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