Find the Missing Term in (a+?)(a+3) = a²+3a+2ab+6b

Polynomial Expansion with Missing Factors

Complete the missing element

(a+?)(a+3)=a2+3a+2ab+6b (a+?)(a+3)=a^2+3a+2ab+6b

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Complete the missing number
00:04 Let's substitute X as the unknown
00:09 Let's properly open parentheses, multiply each factor by each factor
00:29 Let's calculate the products
00:46 Let's compare the corresponding expressions
00:54 Let's reduce what we can
01:00 This is the value of X, now let's check if it fits
01:05 Let's compare the corresponding expressions, substitute the X value we found
01:13 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Complete the missing element

(a+?)(a+3)=a2+3a+2ab+6b (a+?)(a+3)=a^2+3a+2ab+6b

2

Step-by-step solution

To solve this problem, we'll approach it by expanding and comparing terms:

  • Step 1: Apply the distributive property (FOIL method) to expand (a+?)(a+3)(a+?)(a+3).
  • Step 2: Compare each term in the expansion with the corresponding term in a2+3a+2ab+6ba^2 + 3a + 2ab + 6b.
  • Step 3: Identify and solve for the missing element.

Let's execute these steps:

Step 1: Consider the expression (a+b)(a+3)(a+ b)(a+3).

Using distributive property, it expands to:
(a+b)×(a+3)=a×a+a×3+b×a+b×3 (a + b) \times (a + 3) = a \times a + a \times 3 + b \times a + b \times 3 .

This gives us:
=a2+3a+ab+3b = a^2 + 3a + ab + 3b .

Step 2: Now, compare this expansion to the given expression a2+3a+2ab+6ba^2 + 3a + 2ab + 6b.

Step 3: From the comparison, we have:
- a2a^2 terms match.
- 3a3a terms match.
- For the abab term, abab should match 2ab2ab, implying b=2bb = 2b. Therefore, the missing term must contribute an additional bb, making it b+ab=2abb + ab = 2ab. Thus, b=2b = 2.
- For the constant term, 3b=6b3b = 6b, leading to the same conclusion.

Therefore, the solution to the problem is 2b 2b .

3

Final Answer

2b 2b

Key Points to Remember

Essential concepts to master this topic
  • FOIL Method: Expand (a+?)(a+3) using First, Outer, Inner, Last
  • Term Comparison: Match coefficients: ab term needs 2ab, so ? = 2b
  • Verification: Check (a+2b)(a+3) = a²+3a+2ab+6b matches given expression ✓

Common Mistakes

Avoid these frequent errors
  • Assuming the missing term is just b
    Don't just guess b because it appears in the result = wrong expansion! This gives ab + 3b instead of 2ab + 6b. Always expand completely and compare each coefficient systematically.

Practice Quiz

Test your knowledge with interactive questions

\( (x+y)(x-y)= \)

FAQ

Everything you need to know about this question

Why can't the missing term just be b?

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If the missing term were just b, then (a+b)(a+3)=a2+3a+ab+3b (a+b)(a+3) = a^2 + 3a + ab + 3b . But we need 2ab and 6b, so the missing term must be 2b!

How do I know which terms to compare?

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After expanding, match terms with the same variables and powers. Compare a² with a², a terms with a terms, and constant terms with constant terms.

What if I get confused with the FOIL method?

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Remember: First × First, Outer × Outer, Inner × Inner, Last × Last. For (a+2b)(a+3) (a+2b)(a+3) : a×a, a×3, 2b×a, 2b×3.

Can I work backwards from the given expression?

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Yes! Look at the coefficients: if you have 2ab, the missing term contributes a factor of 2 to the b. If you have 6b, that's 3 × 2b.

How do I check if 2b is definitely correct?

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Substitute and expand: (a+2b)(a+3)=a2+3a+2ab+6b (a+2b)(a+3) = a^2 + 3a + 2ab + 6b . This exactly matches the given expression, so 2b is correct!

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