Multivariable Function

Algebra Level 5

As x , y x,y and z z range over the positive real numbers, the maximum value of x y z ( 1 + 5 x ) ( 4 x + 3 y ) ( 5 y + 6 z ) ( z + 18 ) \dfrac{xyz}{(1+5x)(4x+3y)(5y+6z)(z+18)} can be expressed as a b \dfrac{a}{b} , where a a and b b are relatively prime positive integers. Find a + b a+b .


The answer is 5121.

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1 solution

Ritabrata Roy
Mar 10, 2020

Surely,a straightforward application of AM-GM inequality on the expressions in the given parenthesis will not work because it will give a upper bound but not the supremum (the sharpest upper bound) .Hence we need to rearrange the terms sothat we get a solution set (x,y,z) for which the maxima is attained.

I did that same solution too.

Kobe Mercado - 1 year, 2 months ago

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