How can the factor by grouping method simplify polynomials with multiple terms?

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Discussion Overview

The discussion revolves around the factor by grouping method applied to the polynomial expression 8a^3 + 27b^3 + 2a + 3b. Participants explore different approaches to factor the polynomial, examining the validity of their methods and the resulting expressions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a factorization of the polynomial as (4a^2 + 1)(9b^2 + 1)(2a + 3b) but is questioned about the validity of this expression.
  • Another participant suggests a different factorization approach, breaking down the polynomial into (2a + 3b)(4a^2 - 6ab + 9b^2 + 1), claiming it is valid despite the lack of common factors.
  • Several participants inquire about the origin of the +1 in the second participant's final expression, seeking clarification on its derivation.
  • A participant provides an analogy to illustrate the factorization process, comparing it to the expression xy + x = x(y + 1).
  • One participant expresses approval of the work done in the discussion.

Areas of Agreement / Disagreement

There is no consensus on the correct factorization method, as participants present competing views and challenge each other's approaches. The discussion remains unresolved regarding the validity of the proposed factorizations.

Contextual Notes

Participants have not fully addressed the implications of common factors in their proposed factorizations, and there are unresolved questions about the mathematical steps leading to the inclusion of +1 in one of the expressions.

mathdad
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Factor 8a^3 + 27b^3 + 2a + 3b.

8a^3 + 2a + 27b^3 + 3b

2a(4a^2 + 1) = Group A

3b(9b^2 + 1) = Group B

(4a^2 + 1)(9b^2 + 1)(2a + 3b)

Correct?
 
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I would factor as follows:

$$8a^3+27b^3+2a+3b=(2a)^2+(3b)^3+2a+3b=(2a+3b)\left(4a^2-6ab+9b^2\right)+(2a+3b)=(2a+3b)\left(4a^2-6ab+9b^2+1\right)$$

You grouping is valid, however they have no common factors, and so your factored expression isn't equivalent to the original expression.
 
In your final answer, where did + 1 come from?
 
RTCNTC said:
In your final answer, where did + 1 come from?

Think of it like this:

$$(2a+3b)\left(4a^2-6ab+9b^2\right)+(2a+3b)=(2a+3b)\left(4a^2-6ab+9b^2\right)+(2a+3b)\cdot1=(2a+3b)\left(4a^2-6ab+9b^2+1\right)$$

It's the same as writing:

$$xy+x=x(y+1)$$
 
Great work!
 

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