Math Problem Statement

If f(1) = 10 and f'(x) ≥ 2 for 1 ≤ x ≤ 4, how small can f(4) be? (Hint: use MVT)

Solution

This problem involves the Mean Value Theorem (MVT) and understanding how the derivative affects the behavior of the function. Let’s break it down:

Mean Value Theorem (MVT):

The MVT states that if ff is continuous on [a,b][a, b] and differentiable on (a,b)(a, b), then there exists at least one c(a,b)c \in (a, b) such that: f(c)=f(b)f(a)ba.f'(c) = \frac{f(b) - f(a)}{b - a}.

Given Information:

  • f(1)=10f(1) = 10,
  • f(x)2f'(x) \geq 2 for 1x41 \leq x \leq 4.

We need to find the smallest possible value of f(4)f(4).


Step-by-Step Solution:

  1. MVT Application:

    • For ff on the interval [1,4][1, 4], MVT guarantees there exists some c(1,4)c \in (1, 4) such that: f(c)=f(4)f(1)41.f'(c) = \frac{f(4) - f(1)}{4 - 1}.

    • Substituting f(1)=10f(1) = 10: f(c)=f(4)103.f'(c) = \frac{f(4) - 10}{3}.

  2. Bounding f(c)f'(c):

    • Since f(x)2f'(x) \geq 2 for all x[1,4]x \in [1, 4], it follows that: f(4)1032.\frac{f(4) - 10}{3} \geq 2.
  3. Solve for f(4)f(4):

    • Multiply through by 3: f(4)106.f(4) - 10 \geq 6.

    • Add 10 to both sides: f(4)16.f(4) \geq 16.


Final Answer:

The smallest possible value of f(4)f(4) is: 16.\boxed{16}.

Would you like further clarification on MVT or how the inequality was derived? Let me know!


Related Questions:

  1. What is the Mean Value Theorem and how does it apply to differentiable functions?
  2. If f(x)>2f'(x) > 2, how would the minimum value of f(4)f(4) change?
  3. How do we interpret f(x)2f'(x) \geq 2 geometrically for the function f(x)f(x)?
  4. What additional constraints on f(x)f'(x) could impact f(4)f(4)'s minimum value?
  5. Could we use a different theorem (e.g., Rolle's Theorem) in this scenario, and why or why not?

Tip:

Always visualize the problem with a graph when working with derivatives and inequalities—it helps to understand the behavior of the function.

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Math Problem Analysis

Mathematical Concepts

Calculus
Derivatives
Inequalities

Formulas

f'(c) = (f(b) - f(a)) / (b - a)

Theorems

Mean Value Theorem (MVT)

Suitable Grade Level

College Level Calculus