MHB What Is the Minimum Value of $f$ with Given Conditions?

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The discussion focuses on finding the minimum value of the function f, defined as f = √(a² + x²) + √(b² + y²), under the constraints that a, b, c, x, and y are positive real numbers and that x + y = c. Participants suggest using geometric methods to approach the problem. The goal is to minimize f while adhering to the given conditions. The conversation emphasizes the importance of understanding the relationship between the variables to derive the minimum value effectively. The solution involves applying geometric principles to optimize the function.
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find minimum value of $f$
given :
$(1)a,b,c,x,y\in R^+$
$(2)x+y=c$
$(3)f=\sqrt {a^2+x^2}+\sqrt {b^2+y^2}$
find :$min (f)$
 
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My solution:

I use raw calculus (Lagrange optimzation) and I would be happy to see other approaches:

\[f(x,y) = \sqrt{a^2+x^2}+\sqrt{b^2+y^2}\]

The constraint: $g(x,y) = x+y -c = 0$

Solving for $x (>0)$:

\[\frac{\partial f}{\partial x}=-\lambda \frac{\partial g}{\partial x} = -\lambda \: \: \: \wedge \: \: \: \frac{\partial f}{\partial y}=-\lambda \frac{\partial g}{\partial y} = -\lambda\]

\[ \Rightarrow \frac{x}{\sqrt{a^2+x^2}} = \frac{y}{\sqrt{b^2+y^2}}=\frac{c-x}{\sqrt{b^2+(c-x)^2}}\]

\[ \Rightarrow x^2(b^2+(c-x)^2)=(c-x)^2(a^2+x^2)\]

\[ \Rightarrow (b^2-a^2)x^2+2a^2cx-a^2c^2=0\]

\[ \Rightarrow x = \frac{ac}{a+b} \Rightarrow y = c-x = \frac{bc}{a+b}\]

- and the minimum value of f is: $f_{min} = \sqrt{(a+b)^2+c^2}$.
 
Albert said:
find minimum value of $f$
given :
$(1)a,b,c,x,y\in R^+$
$(2)x+y=c$
$(3)f=\sqrt {a^2+x^2}+\sqrt {b^2+y^2}---(1)$
find :$min (f)$
using geometry
consturct $\triangle ACD,\angle A=90^o, \overline {AC}=a+b=\overline {AB}+\overline {BC}$
$\overline {DA}=c=\overline {DQ}+\overline {QA}=x+y$
point $P$ is an inner point of $\triangle ACD,\overline {PQ}=a=\overline {AB},$ and $\overline {PQ}\perp \overline{AD}$
$\overline {PB}=y=\overline {AQ},$ and $\overline {PB}\perp \overline{AC}$
we have $(1):f=\overline {CP}+\overline {PD}\geq\overline {CD}=\sqrt{(a+b)^2+c^2}$
equality holds when point $P $ locates on $\overline {CD}$
 
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