# Inequality of arithmetic and geometric means

1. Jan 12, 2013

### Numeriprimi

Hey! I have this: 2(√(1-a^2 ))+ 2a
How to determine the maximum value of this?

I think good for this is Inequality of arithmetic and geometric means, but I don't know how use this, because I don't calculate with this yet.

So, have you got any ideas?

Poor Czech Numeriprimi... If you don't understand my primitive English, write, I will try to write this better ;-)

2. Jan 12, 2013

### symbolipoint

Could be a Calculus derivative problem. Does this f(a)=2(√(1-a^2 ))+ 2a have a maximum or minimum?

3. Jan 12, 2013

### HallsofIvy

Carrying that farther $f(a)= 2(1- a^2)^{1/2}+ 2a$ has derivative $f'(a)= -2(1- a^2)^{-1/2}+ 2$. Any maximum (or minimum) must occur where that derivative is 0 or does not exist. It does not exist when a= 1 or -1 because in that case we have a 0 in the denominator. It is 0 when $\sqrt{1- a^2}= 1$ or a= 0.

Check the values of $2\sqrt{1- a^2}+ 2a$ at x= -1, 0, and 1 to see which is the maximum value.

Another way to look at this is to see that if $y= 2\sqrt{1- a^2}+ 2a$ then $y- 2a= 2\sqrt{1- a^2}$ and so $y^2- 4ay+ 4a^2= 4- 4a^2$ or $8a^2- 4ay+ y^2= 4$ which is the graph of an ellipse with major and minor axes rotated from the coordinate axes.

4. Jan 12, 2013

### Numeriprimi

Please, derivate no... I still have a few years time to learn this.
I need to use Inequality of arithmetic and geometric means, bud how?

5. Jan 12, 2013

### AlephZero

Your expression has two parts, $2\sqrt{1-a^2}$ amd $2a$.

If you are supposed to use arithmetic and geometric means, you need to find some quantities that give those arithmetic and geometric means.

For example if the quantities are $x$ and $y$ and the arithmetic mean is $2a$, you have the equation $(x+y)/2 = 2a$

And you have another equation for the geometric mean .....

6. Jan 13, 2013

### Numeriprimi

Yes, i have this: n=4
And what now?