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ID:
Class: 12
Subject: Math
Topic: Application of Derivative
Type: Long
Question:
Find two positive numbers whose sum is 16 and the sum of whose cubes is minimum.
Official Solution
Explanation:
Let one number be \( x \).
Then the other number is \( 16 - x \).
We want to minimize the sum of their cubes:
\[
S(x) = x^3 + (16 - x)^3
\]
Differentiate:
\[
S'(x) = 3x^2 - 3(16 - x)^2
\]
Set \( S'(x) = 0 \) for critical points:
\[
3x^2 = 3(16 - x)^2 \Rightarrow x^2 = (16 - x)^2
\]
Expand:
\[
x^2 = 256 - 32x + x^2 \Rightarrow -32x + 256 = 0
\Rightarrow x = \frac{256}{32} = 8
\]
Now check second derivative:
\[
S''(x) = 6x + 6(16 - x) = 6x + 96 - 6x = 96 > 0
\]
Since \( S''(8) > 0 \), this confirms a local minimum.
\[\] Final Answer:
\[
\boxed{\text{The two numbers are } 8 \text{ and } 16-8=8.}
\]
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