Sigma Percentile
JEE Main 2018 (15 April Evening)
LEVELJEE Main

Animated Solution for Mathematics - Binomial Theorem: The coefficient of in the expansion of is equal to :-

Select Answer:

Visualized Solution

Understanding the Problem

  • Given expression:
  • Objective: Find the coefficient of in the expansion of .

Expanding

  • Available powers of :
  • Corresponding coefficients:

Expanding

  • Available powers of :
  • Corresponding coefficients:

Expanding

  • Available powers of : (ignoring )
  • Corresponding coefficients:

The Selection Rule

  • Let powers chosen be from the three factors respectively.
  • Condition:
  • Constraints: , ,

Case 1: Choosing from

  • Case 1: (from )
  • Equation:
  • Solution: (since )
  • Coefficient:

Case 2: Choosing from (Part A)

  • Case 2: (from )
  • Equation:
  • Subcase 2.1:
  • Coefficient:

Case 2: Choosing from (Part B)

  • Case 2 continued:
  • Subcase 2.2:
  • Coefficient:

Case 3: Choosing from

  • Case 3: (from )
  • Equation:
  • Solution:
  • Coefficient:

Summing the Coefficients

  • Total Coefficient =
  • Total Coefficient =

Final Answer and Takeaway

  • Final Answer:
  • Key Takeaway: For products of multiple polynomials, use combinatorial selection of powers to find specific coefficients.

The Sigma Insight: General Term and Middle Term

Solution Diagram

Analyzing the Setup

Imagine you are standing before a massive, intimidating wall of algebra: . The question asks for the coefficient of .
In the world of JEE Advanced, brute force is rarely the path to victory. The path to victory is elegance. We will navigate this maze by focusing only on the terms that contribute to our target power.

The Three Baskets

Think of this expression as three distinct baskets. To form the final expansion, you must reach into each basket, pull out exactly one term, and multiply them together.
The first basket, , expands to . The powers available here are .
The second basket, , expands to . The powers available here are .
The third basket, , expands to . The powers available here are (we ignore as it exceeds our target).

The Constraint Equation

We need the sum of the exponents to be . Let be the exponent chosen from the first basket, from the second, and from the third.
Our mission is to solve the equation:
Subject to the constraints:

The Systematic Hunt

We will fix from the third basket because it has the largest steps, which limits our search space significantly.
Case 1: Let . The equation becomes . Given , the only solution is and . The coefficient is .
Case 2: Let . The equation becomes . If , then . The coefficient is . If , then . The coefficient is .
Case 3: Let . The equation becomes . Since must be even, must be , which forces . The coefficient is .

Final Calculation

We have exhausted all possibilities to construct . We now sum the coefficients:
We didn't expand a single complex polynomial. We used logic, constraints, and systematic counting to arrive at the final answer of 52.

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