Wow what a crazy thing

Calculus Level 3

When the function x 1 x x \sqrt{1-x} takes its maximum value for 0 x 1 0 \leq x \leq 1 , what is x x ?

Give your answer to 3 decimal places.


Bonus: Generalize this for the function x n ( 1 x ) m x^n (1-x)^m .


The answer is 0.6666666666666.

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1 solution

Md Zuhair
Mar 21, 2017

Easy one,

First of all we'll try generalization.

So y = x n × 1 x m y = x^n \times {1-x}^m

So d y d x = n x n 1 × ( 1 x ) m m ( 1 x ) m 1 x n \dfrac{dy}{dx} = nx^{n-1} \times (1-x)^m - m(1-x)^{m-1} x^n

Now we know for Maxima or Minima , We have d y d x = 0 \dfrac{dy}{dx} = 0

So We get 0 = n x n 1 × ( 1 x ) m m ( 1 x ) m 1 x n 0 = nx^{n-1} \times (1-x)^m - m(1-x)^{m-1} x^n

So On simplyfying we get,

m n ( 1 x ) = x \dfrac{m}{n} (1-x) = x

or , n n x = m x n - nx = mx

OR x = n m + n x = \dfrac{n}{m+n}

So General Term for x = n m + n \boxed{x= \dfrac{n}{m+n}}

SO for this problem we get m = 1 2 m = \dfrac{1}{2} and n = 1 n = 1

Hence Putting m and n we get,

x = 2 3 = 0.666... x = \dfrac{2}{3} = 0.666...

So Value of x such that y = x 1 × 1 x 1 2 y = x^1 \times {1-x}^{\dfrac{1}{2}} in minimum will be

x = 2 3 = 0.666... \boxed{x = \dfrac{2}{3} = 0.666...}

A cool result. I believe it also works for multiple variables, for example, the maximum of

x^2 * y^3 * z^4

when

x+y+z=9

is 27648 when x=2, y=3, and z=4.

Alex Li - 4 years, 2 months ago

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I think yes. It may work

Md Zuhair - 4 years, 2 months ago

There are some careless mistakes in your solution but I think you know where they are right?

Noah Hunter - 4 years, 2 months ago

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Got it. Will rectify in my laptop, now using phone , so not possible

Md Zuhair - 4 years, 2 months ago

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