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8. Calculus
Finding terms independent of x in an expansion

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Finding Terms Independent of \( x \) in an Expansion

Introduction

Understanding how to identify terms independent of \( x \) in a binomial expansion is crucial for mastering the Cambridge IGCSE Mathematics - Additional (0606) syllabus. This skill not only enhances problem-solving abilities but also lays the foundation for more advanced mathematical concepts within the unit on Series.

Key Concepts

Binomial Theorem Overview

The Binomial Theorem provides a systematic method for expanding expressions of the form \( (a + b)^n \), where \( n \) is a non-negative integer. The theorem states: $$ (a + b)^n = \sum_{k=0}^{n} \binom{n}{k} a^{n-k} b^{k} $$ Here, \( \binom{n}{k} \) represents the binomial coefficient, calculated as: $$ \binom{n}{k} = \frac{n!}{k!(n - k)!} $$ This expansion results in \( n + 1 \) terms, each comprising combinations of \( a \) and \( b \) raised to varying powers.

General Term in Binomial Expansion

The \( k^{th} \) term (\( T_{k+1} \)) in the expansion of \( (a + b)^n \) is given by: $$ T_{k+1} = \binom{n}{k} a^{n-k} b^{k} $$ This general term formula is pivotal for identifying specific terms within the expansion, such as those independent of \( x \).

Terms Independent of \( x \)

In many binomial expansions, variables are present, and certain terms may be free of specific variables like \( x \). To find terms independent of \( x \), follow these steps:
  1. Identify the Expansion: Determine the binomial expression and the value of \( n \).
  2. Express in Terms of \( x \): Rewrite the binomial so that one of the terms contains \( x \).
  3. Apply the General Term Formula: Use the general term formula to express the \( k^{th} \) term.
  4. Set Exponent of \( x \) to Zero: For a term to be independent of \( x \), the exponent of \( x \) must be zero.
  5. Solve for \( k \): Determine the value of \( k \) that satisfies the condition.
  6. Find the Specific Term: Substitute \( k \) back into the general term formula to find the term independent of \( x \).

Example Problem

Find the term independent of \( x \) in the expansion of \( (2x + \frac{3}{x^2})^5 \).
  1. Identify the Expansion: \( (2x + \frac{3}{x^2})^5 \)
  2. Express in Terms of \( x \): Already in the required form.
  3. General Term: \( T_{k+1} = \binom{5}{k} (2x)^{5-k} \left(\frac{3}{x^2}\right)^k \)
  4. Set Exponent of \( x \) to Zero: \( (5 - k) + (-2k) = 0 \Rightarrow 5 - 3k = 0 \Rightarrow k = \frac{5}{3} \)
Since \( k \) must be an integer, there is no term independent of \( x \) in this expansion.

Common Mistakes

  • Incorrectly setting up the equation for the exponents of \( x \).
  • Forgetting to simplify the exponents before solving for \( k \).
  • Assuming that a non-integer value of \( k \) is possible.

Practice Problems

  1. Find the term independent of \( x \) in the expansion of \( \left(\frac{x}{2} + 3\right)^4 \).
  2. Determine the term independent of \( x \) in \( (5x^3 - 2)^6 \).
  3. In the expansion of \( (1 + x)^n \), find \( n \) such that there's exactly one term independent of \( x \).

Solutions to Practice Problems

  1. Solution: \( \left(\frac{x}{2} + 3\right)^4 \)
    1. General Term: \( T_{k+1} = \binom{4}{k} \left(\frac{x}{2}\right)^{4-k} (3)^k \)
    2. Exponent of \( x \): \( 4 - k = 0 \Rightarrow k = 4 \)
    3. Term: \( \binom{4}{4} \left(\frac{x}{2}\right)^0 (3)^4 = 1 \times 1 \times 81 = 81 \)
    Thus, the term independent of \( x \) is \( 81 \).
  2. Solution: \( (5x^3 - 2)^6 \)
    1. General Term: \( T_{k+1} = \binom{6}{k} (5x^3)^{6-k} (-2)^k \)
    2. Exponent of \( x \): \( 3(6 - k) = 0 \Rightarrow 18 - 3k = 0 \Rightarrow k = 6 \)
    3. Term: \( \binom{6}{6} (5x^3)^0 (-2)^6 = 1 \times 1 \times 64 = 64 \)
    Thus, the term independent of \( x \) is \( 64 \).
  3. Solution: \( (1 + x)^n \)
    1. General Term: \( T_{k+1} = \binom{n}{k} (1)^{n-k} (x)^k = \binom{n}{k} x^k \)
    2. Exponent of \( x \): \( k = 0 \)
    3. For exactly one term independent of \( x \), only \( k = 0 \) satisfies the condition.
    4. Hence, for any \( n \), there is exactly one term independent of \( x \).

Advanced Concepts

Theoretical Foundations

Delving deeper into the identification of terms independent of \( x \), it is essential to understand the interplay between exponents in the binomial expansion. Suppose we have an expression \( (ax^m + b)^n \), where \( a \), \( b \), and \( m \) are constants. The general term in its expansion is: $$ T_{k+1} = \binom{n}{k} (ax^m)^{n-k} b^{k} = \binom{n}{k} a^{n-k} b^{k} x^{m(n-k)} $$ To find the term independent of \( x \), set the exponent of \( x \) to zero: $$ m(n - k) = 0 \Rightarrow k = n $$ Therefore, the term independent of \( x \) is: $$ T_{n+1} = \binom{n}{n} a^{n-n} b^{n} = b^{n} $$ This theoretical derivation shows that in cases where the exponent of \( x \) can only reach zero when \( k = n \), the independent term is straightforward to find.

Mathematical Derivation

Consider the binomial expansion \( (a + b)^n \). The general term is: $$ T_{k+1} = \binom{n}{k} a^{n-k} b^k $$ If \( a \) or \( b \) contains a variable \( x \), say \( a = p x^m \), then: $$ T_{k+1} = \binom{n}{k} (p x^m)^{n-k} b^k = \binom{n}{k} p^{n-k} b^k x^{m(n - k)} $$ To find the term independent of \( x \), set \( m(n - k) = 0 \), which implies: $$ k = n $$ Thus, the independent term is: $$ T_{n+1} = \binom{n}{n} p^{n-n} b^n = b^n $$ This derivation confirms that under specific conditions, the term independent of \( x \) can be directly identified without extensive calculations.

Complex Problem-Solving

**Problem:** Find the term independent of \( x \) in the expansion of \( \left( \frac{3x^2}{4} - \frac{5}{x} \right)^7 \). **Solution:**
  1. Express the Binomial: \( \left( \frac{3x^2}{4} - \frac{5}{x} \right)^7 \)
  2. General Term: $$ T_{k+1} = \binom{7}{k} \left( \frac{3x^2}{4} \right)^{7-k} \left( -\frac{5}{x} \right)^k $$
  3. Simplify the Term: $$ T_{k+1} = \binom{7}{k} \left( \frac{3^{7-k}}{4^{7-k}} \right) x^{2(7 - k)} \left( -5^k \right) x^{-k} = \binom{7}{k} \frac{3^{7-k} (-5)^k}{4^{7 - k}} x^{14 - 3k} $$
  4. Set Exponent of \( x \) to Zero: $$ 14 - 3k = 0 \Rightarrow 3k = 14 \Rightarrow k = \frac{14}{3} $$
  5. Interpretation: Since \( k \) must be an integer (as it represents the term number in the expansion), there is no term independent of \( x \) in this expansion.
**Conclusion:** No term independent of \( x \) exists in the expansion of \( \left( \frac{3x^2}{4} - \frac{5}{x} \right)^7 \).

Interdisciplinary Connections

The concept of identifying terms independent of a variable in a binomial expansion has applications beyond pure mathematics. In physics, for instance, this technique can be used to simplify equations in mechanics or thermodynamics where variables represent physical quantities like force or temperature. In engineering, it aids in simplifying expressions related to stress-strain relationships or in electrical engineering for analyzing circuit components. Furthermore, in economics, understanding how to isolate specific terms within a growth model can help in analyzing factors that remain constant while others vary, thereby aiding in more accurate forecasting and modeling.

Advanced Practice Problems

  1. Determine the term independent of \( x \) in the expansion of \( \left( \frac{2x^3}{5} + \frac{4}{x^2} \right)^8 \).
  2. Find all values of \( n \) for which the expansion of \( (x + \frac{1}{x})^n \) contains exactly two terms independent of \( x \).
  3. In the expansion of \( \left( 7x - \frac{9}{x^3} \right)^{10} \), find the term that is independent of \( x \).

Solutions to Advanced Practice Problems

  1. Solution: \( \left( \frac{2x^3}{5} + \frac{4}{x^2} \right)^8 \)
    1. General Term: $$ T_{k+1} = \binom{8}{k} \left( \frac{2x^3}{5} \right)^{8 - k} \left( \frac{4}{x^2} \right)^k = \binom{8}{k} \frac{2^{8 - k} 4^k}{5^{8 - k}} x^{3(8 - k) - 2k} = \binom{8}{k} \frac{2^{8 - k} 4^k}{5^{8 - k}} x^{24 - 5k} $$
    2. Set Exponent of \( x \) to Zero: $$ 24 - 5k = 0 \Rightarrow 5k = 24 \Rightarrow k = \frac{24}{5} $$
    3. Interpretation: No integer \( k \) satisfies the equation, hence no term is independent of \( x \) in this expansion.
    **Conclusion:** No term independent of \( x \) exists.
  2. Solution: \( (x + \frac{1}{x})^n \) **To have two terms independent of \( x \):** The exponents of \( x \) in the general term are \( n - 2k \). To have independent terms, set \( n - 2k = 0 \). For two independent terms, there must be two distinct values of \( k \) satisfying this condition. However, \( n - 2k = 0 \) implies \( k = \frac{n}{2} \). This has only one solution unless \( n \) is even, allowing \( k = \frac{n}{2} \). Therefore, only one term is independent of \( x \) when \( n \) is even. For two independent terms, it's impossible in this scenario. **Conclusion:** There are no values of \( n \) for which the expansion contains exactly two terms independent of \( x \).
  3. Solution: \( \left( 7x - \frac{9}{x^3} \right)^{10} \)
    1. General Term: $$ T_{k+1} = \binom{10}{k} (7x)^{10 - k} \left( -\frac{9}{x^3} \right)^k = \binom{10}{k} 7^{10 - k} (-9)^k x^{10 - k - 3k} = \binom{10}{k} 7^{10 - k} (-9)^k x^{10 - 4k} $$
    2. Set Exponent of \( x \) to Zero: $$ 10 - 4k = 0 \Rightarrow 4k = 10 \Rightarrow k = \frac{10}{4} = 2.5 $$
    3. Interpretation: Since \( k \) must be an integer, there is no term independent of \( x \) in this expansion.
    **Conclusion:** No term independent of \( x \) exists.

Comparison Table

Aspect Basic Concepts Advanced Concepts
Definition Identifying terms in a binomial expansion. Deriving conditions for terms independent of variables and exploring complex expansions.
Application Solving straightforward binomial expansion problems. Handling multi-variable expansions and connecting to other mathematical fields.
Difficulty Level Basic identification and calculation. In-depth proofs, derivations, and interdisciplinary applications.
Tools Used Binomial coefficients and exponent rules. Advanced algebraic techniques and theoretical mathematics.

Summary and Key Takeaways

  • Mastering the identification of terms independent of \( x \) enhances problem-solving skills.
  • The Binomial Theorem is essential for expanding and analyzing binomial expressions.
  • Advanced concepts involve deeper theoretical understanding and interdisciplinary applications.
  • Practice with varied problems reinforces the ability to handle complex expansions.

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Examiner Tip
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Tips

To excel in identifying terms independent of \( x \), always **double-check the exponents** in your general term. A useful mnemonic is **"Set x to Zero"**, reminding you to equate the total exponent of \( x \) to zero. Additionally, practicing with a variety of problems enhances your ability to recognize patterns and apply the Binomial Theorem effectively, which is essential for securing top marks in your IGCSE exams.

Did You Know
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Did You Know

The concept of identifying terms independent of variables in binomial expansions is not just limited to mathematics. In **chemistry**, similar principles are used to balance chemical equations by ensuring the number of atoms for each element remains constant. Additionally, in **computer science**, algorithms that parse mathematical expressions often implement rules to identify and simplify terms independent of certain variables, enhancing computational efficiency.

Common Mistakes
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Common Mistakes

Students often make errors when finding terms independent of \( x \). A frequent mistake is **incorrectly setting up the equation for exponents**, leading to wrong values of \( k \). For example, in the expansion of \( (2x + 3)^5 \), some might set \( 5 - k = 0 \) ignoring the \( x \) term's exponent. The correct approach is to consider the exponents of \( x \) from both terms and set their sum to zero: \( (5 - k) \times 1 + (0) \times k = 5 - k = 0 \), thus \( k = 5 \).

FAQ

What is the Binomial Theorem?
The Binomial Theorem provides a formula for expanding expressions of the form \( (a + b)^n \) into a sum involving terms of the form \( \binom{n}{k} a^{n-k} b^k \).
How do you find the general term in a binomial expansion?
The general term \( T_{k+1} \) in the expansion of \( (a + b)^n \) is given by \( \binom{n}{k} a^{n-k} b^k \).
Why must \( k \) be an integer when finding terms independent of \( x \)?
In the binomial expansion, \( k \) represents the term number and must be a whole number. Non-integer values of \( k \) do not correspond to actual terms in the expansion.
Can there be more than one term independent of \( x \) in a binomial expansion?
Generally, there is only one term independent of \( x \) in a binomial expansion, unless specific conditions allow for multiple solutions, which is rare.
What are some common applications of finding terms independent of a variable?
This skill is useful in physics for simplifying equations, in engineering for analyzing systems, and in economics for isolating constant factors in growth models.
8. Calculus
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