Summary of Inequalities 2 | CAT Preparation 2024 | Algebra | Quantitative Aptitude

Summary of "Inequalities 2 | CAT Preparation 2024 | Algebra | Quantitative Aptitude"

This video by Ravi Prakash continues the discussion on inequalities, focusing on problems involving the minimum and maximum values of products of variables under sum constraints. It is tailored for CAT exam preparation and covers conceptual clarity, problem-solving techniques, and strategic thinking for Quantitative Aptitude.

Main Ideas and Concepts

• Problem Setup:
  • Given \( a + b + c = 30 \), find the minimum and maximum values of the product \( a \times b \times c \) under different conditions on \( a, b, c \):
    • Case 1: \( a, b, c \) are real numbers (no sign restriction).
    • Case 2: \( a, b, c \) are positive real numbers.
    • Case 3: \( a, b, c \) are distinct natural numbers.
• Case 1: Real Numbers (no restriction)
  • Since \( a, b, c \) can be any real numbers, the product \( a \times b \times c \) can be infinitely large or infinitely small.
  • Minimum value: \(-\infty\)
  • Maximum value: \(+\infty\)
  • Explanation: By choosing large positive or negative values for variables while keeping the sum constant, the product can tend to infinity or negative infinity.
• Case 2: Positive Real Numbers
  • The product \( a \times b \times c \) is maximized when \( a, b, c \) are equal (or as close as possible) due to the AM-GM inequality.
  • Maximum product: \(10 \times 10 \times 10 = 1000\) (since \(a + b + c = 30\))
  • Minimum product: Approaches 0 (tends to zero but never exactly zero since all are positive).
  • Explanation: For fixed sum, product is maximized when numbers are equal; minimized when one number tends to zero.
• Case 3: Distinct Natural Numbers
  • Since \( a, b, c \) are distinct natural numbers summing to 30, the product is maximized when numbers are as close as possible but distinct.
  • Maximum product example: \(9 \times 10 \times 11 = 990\)
  • Minimum product example: \(1 \times 2 \times 27 = 54\)
  • Explanation: Distinctness restricts equality; closest distinct numbers near equal partition maximize the product, while minimum is achieved by spreading values far apart.
• Extension to Four Variables \(a, b, c, d\) with product constraint
  • Problem: Given \( a \times b \times c \times d = 10! \) (factorial of 10), find the minimum value of \( a + b + c + d \) where \( a, b, c, d \) are natural numbers.
  • Key insight: For fixed product, sum is minimized when numbers are equal or as close as possible.
  • Approach:
    • Factorize \(10! = 2^8 \times 3^4 \times 5^2 \times 7\).
    • Take the 4th root of \(10!\) to estimate each number \(a, b, c, d\).
    • Approximate each number around 43.
    • Assign prime factors to four numbers to keep them close to 43.
    • Example assignment:
      • \(a = 7 \times 3 \times 2 = 42\)
      • \(b = 5 \times 3 \times 3 = 45\)
      • \(c = 5 \times 2 \times 2 \times 2 = 40\)
      • \(d = 2 \times 2 \times 2 \times 2 \times 3 = 48\)
    • Sum: \(42 + 45 + 40 + 48 = 175\) (minimum sum)
  • Lesson: Prime factorization and distribution is crucial to find numbers close to the root value.
• Another problem involving four variables \(a, b, c, d\) with the equation:

  \[ (6 - a)(6 - b)(6 - c)(6 - d) = 45 \]

  • \(a, b, c, d\) are distinct natural numbers.
  • Goal: Find minimum and maximum values of \(a + b + c + d\).
  • Constraints:
    • Since \(a, b, c, d\) are natural numbers, \(6 - a, 6 - b, 6 - c, 6 - d\) cannot be zero or negative in a way that \( ...

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