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Sunday, 6 September 2015

Find \max xy^3z^7 where x+y+z=1

As a brief introduction, 'problem of the day' posts will be about mathematical thinking (problem-solving and proofs). The questions and solutions are extracted from online sources. I will add alternative solutions and mention something interesting about the problems if I can.

Today's problem:

Find the maximum of xy^3z^7 with non-negative real numbers in the plane x+y+z=1.



We make use of AM-GM inequality: 1=x+y+z=x+\frac{1}{3}y+\frac{1}{3}y+\frac{1}{3}y+\frac{1}{7}z+\cdots+\frac{1}{7}z\geq 11 \sqrt[11]{x(\frac{1}{3}y)^3(\frac{1}{7}z)^7}.
To obtain the maximum, we want to find x,y,z for the equality case. Namely, x=\dfrac{1}{3}y=\dfrac{1}{7}z=\dfrac{1}{11}. We thus have the maximum xy^3z^7=\dfrac{3^3 7^7}{11^{11}}.

Alternatively, we can use lagrange multiplier since this is an optimisation problem with a constraint. Let F(x,y,z,\lambda)=xy^3z^7-\lambda(x+y+z-1). Then, we have a system of equations \begin{cases}F_x=y^3z^7-\lambda=0\\F_y=3xy^2z^7-\lambda=0\\F_z=7xy^3z^6-\lambda=0\\ x+y+z=1.\end{cases}
From the first three equations, we know y=3x,z=7x. Therefore, substituting them into the last equation, we have x+3x+7x=1 and thus x=\dfrac{1}{11},y=\dfrac{3}{11},z=\dfrac{7}{11}.

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