Math Problem Statement

(a,b) ∈ R^2 and c > 0, where |a| < c and |b| < c. Prove that |(a + b) / 2| < c.

Solution

The image contains a mathematical inequality involving absolute values. Here’s a breakdown of the steps:

  1. Given:

    • (a,b)R(a, b) \in \mathbb{R} and c>0c > 0
    • a<c|a| < c and b<c|b| < c
  2. Goal:

    • To prove that a+b2<c\left| \frac{a + b}{2} \right| < c

Proof Strategy:

  • Since a<c|a| < c, we know that c<a<c-c < a < c.
  • Similarly, b<c|b| < c implies c<b<c-c < b < c.

Now, adding these inequalities gives:

c+(c)<a+b<c+c-c + (-c) < a + b < c + c 2c<a+b<2c-2c < a + b < 2c

Dividing the entire inequality by 2:

c<a+b2<c-c < \frac{a + b}{2} < c

Thus, a+b2<c\left| \frac{a + b}{2} \right| < c, which proves the required result.


Would you like further details on this proof or have any questions?

Here are 5 related questions to expand your understanding:

  1. How would this proof change if c0c \leq 0?
  2. What happens if we consider more variables, say a<c1|a| < c_1 and b<c2|b| < c_2?
  3. How does this inequality relate to the concept of the midpoint between two numbers?
  4. Can we generalize this result for more than two variables?
  5. How would you handle the case when aa or bb are complex numbers?

Tip: When working with absolute value inequalities, breaking them down into their equivalent compound inequalities can simplify the proof process.

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Math Problem Analysis

Mathematical Concepts

Inequalities
Absolute Values

Formulas

If |a| < c, then -c < a < c
If |b| < c, then -c < b < c
Sum of inequalities: -2c < a + b < 2c
Divide by 2: -c < (a + b) / 2 < c

Theorems

Inequality properties
Absolute value properties

Suitable Grade Level

Grade 9-12