Inspired by Rohit Kumar

Algebra Level 2

As x x ranges over all real values, what is the minimum of

x 2 + ( x + 1 ) 2 ? x^2 + ( x + 1) ^2 ?

0 0 1 4 \frac{1}{4} 1 2 \frac{1}{2} 1 1

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16 solutions

Alex Delhumeau
May 31, 2015

f ( x ) = x 2 + ( x + 1 ) 2 = 2 x 2 + 2 x + 1 f(x)=x^2+(x+1)^2=2x^2+2x+1 and its vertex is b 2 a = 1 2 -\frac{b}{2a}=-\frac{1}{2} . Therefore, the minimum is f ( 1 2 ) f(-\frac{1}{2}) ,

\implies 1 2 \large{\boxed{\frac{1}{2}}} .

Moderator note:

How do you know that it's a minimum value rather than a maximum value?

reply to challenge masternote.....because the coefficient of x2 is positive,,,,indicating a smiling face graph..therefore you know it is minimum....

Zack Yeung - 6 years ago

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the smiling face graph part killed me ^^

Patrick Bourg - 6 years ago

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I thought "smiling" was common terminology.

Jake Lai - 6 years ago

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@Jake Lai well, it is more that it is not very accurate or well-defined! turning the graph upside down still makes the function convex, but it is now 'sad'! (no troll intended) . Also, there are lots of functions which are convex but hardly look like any nice smiling smiley!!

Patrick Bourg - 6 years ago

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@Patrick Bourg turning it down its x2 coefficient would be negative....its the simplest way and most accurate way to describe the shape of graph...lol

Zack Yeung - 6 years ago

isn't it the formula for finding the minimum value is (4ac - b^2 )/4a?

Ojna Eugitaraba - 6 years ago

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there are alot of ways....but i never heard this...and by looking at the coeeficient would be way faster..

Zack Yeung - 6 years ago

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but if you are just looking for the coefficient, you are just identifying if it is minimum or maximum, you must use the above formula to get the value of minimum or maximum..

Ojna Eugitaraba - 6 years ago

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@Ojna Eugitaraba just differentiate and let dy/dx=0 and sub back to equation or simply completing the square....as i mentioned...there are a lot of ways to get things done.....i don't memorize formula..so....my twocents

Zack Yeung - 6 years ago

That formula is derived from the above method. When you find the x-coordinate of the vertex with -b/2a and plug it into the original quadratic, you get that formula.

Alex Bean - 5 years, 11 months ago

I also did the same :)

Vishal S - 5 years, 11 months ago

A good rule of thumb is that when the degree of your polynomial is even and the coefficient of the dominant variable (the variable with the greatest power) is positive that means it will provide the largest contribution to function. This means that since as you greater and greater values of x x then the function will naturally tend toward being arbitrary large (this goes both ways as very large negative numbers will be large positive numbers since squaring a negative makes positive). This means that naturally there is a a minimum, mind you it does not tell you how many minimums there are in the function, just that there has to be one.

Cris Kovacs - 5 years, 6 months ago

the parabola is opened upwards as coeffecient of a is positive so it will only have a mimima and its global maxima is infinte

akhil sai - 4 years, 1 month ago
Prakhar Gupta
Jun 3, 2015

x 2 + ( x + 1 ) 2 = 2 x 2 + 2 x + 1 x^{2} + (x+1)^{2} = 2x^{2}+2x+1 = 2 ( x + 1 2 ) 2 + 1 2 = 2\Big( x+\dfrac{1}{2}\Big)^{2} + \dfrac{1}{2} The minimum value is achieved when the term which is squared is 0 0 . Hence the minimum value is 1 2 \dfrac{1}{2} .

Lucas Tell Marchi
May 31, 2015

Let f f be the function such that f ( x ) = x 2 + ( x + 1 ) 2 f(x) = x^{2} + (x+1)^{2} . Then the first derivative is

f ( x ) = 4 x + 2 f'(x) = 4x + 2

But f ( x ) = 0 f'(x) = 0 gives is an extreme point at x = 1 / 2 x = -1/2 . The second derivative is

f ( x ) = 4 > 0 f''(x) = 4 > 0

But f ( x ) > 0 f''(x) > 0 means that the point we chose is a minimum. Then the minimum value of the function f f happens at x = 1 / 2 x = -1/2 and

f ( 1 / 2 ) = 1 2 f(-1/2) = \frac{1}{2}

Moderator note:

Yes, this is the calculus approach to this problem. But why is this statement true?

But f ( x ) > 0 f''(x) > 0 means that the point we chose is a minimum.

Since this is just an algebra problem, can you solve this without resorting to calculus?

The simplest way to solve this without relying on calculus is to use the method of completing the square. If we consider the given expression as f ( x ) f(x) , then we have,

f ( x ) = x 2 + ( x + 1 ) 2 = 2 x 2 + 2 x + 1 = 2 [ x 2 + x + 1 2 ] = 2 [ ( x + 1 2 ) 2 + 1 2 1 2 2 ] = 2 [ ( x + 1 2 ) 2 + 1 4 ] \begin{aligned}f(x)=x^2+(x+1)^2&=2x^2+2x+1\\&=2\left[x^2+x+\frac 12\right]\\&=2\left[\left(x+\frac{1}{2}\right)^2+\frac 12-\frac{1}{2^2}\right]=2\left[\left(x+\frac{1}{2}\right)^2+\frac{1}{4}\right]\end{aligned}

Now, we have, by the trivial inequality that,

x R , ( x + 1 2 ) 2 0 f ( x ) 2 1 4 = 1 2 \forall~x\in\Bbb R~,~\left(x+\frac 12\right)^2\geq 0\iff f(x)\geq 2\cdot\frac 14=\frac 12

with equality iff x + 1 2 = 0 x+\dfrac{1}{2}=0 , i.e., iff x = 1 2 x=-\dfrac{1}{2} .

Prasun Biswas - 6 years ago

In response to the first part of the Challenge Master note, since f ( x ) > 0 f''(x) > 0 at the stationary point x = 1 / 2 x = -1/2 , f ( 1 2 ) = 1 2 f(-\frac {1}{2}) = \frac {1}{2} is a minimum of the function by the Second Derivative Test .

One can also graphically discern this, since f ( x ) > 0 f''(x) > 0 means the function is concave up . This, in conjunction with the fact that there is a stationary point at x = 1 2 x = \frac {1}{2} allows for a visualization of the minimum at f ( 1 2 ) = 1 2 f(-\frac {1}{2}) = \frac {1}{2} .

The same result can be obtained using the First Derivative Test as well.

Willy Yang - 5 years, 1 month ago

f ( x ) = y = x 2 + ( x + 1 ) 2 = 2 x 2 + 2 x + 1 f(x)=y=x^2+(x+1)^2=2x^2+2x+1

d y d x = 4 x + 2 \frac{dy}{dx}=4x+2

You obtain your max. or min. points if there are any at d y d x = 0 \frac{dy}{dx}=0

Then, 4 x + 2 = 0 , x = 1 2 4x+2=0, \boxed{x=-\frac12}

Now substitute by x x value in f ( x ) f(x)

You get y = 1 2 \boxed{y=\frac12}

Note: this a minimum value because f ( x ) > 0 f''(x)>0 (a positive number)

Daniel Liu
Jun 3, 2015

Substitute y = x + 1 2 y=x+\dfrac{1}{2} to get ( y 1 2 ) 2 + ( y + 1 2 ) 2 = 2 y 2 + 1 2 \left(y-\dfrac{1}{2}\right)^2+\left(y+\dfrac{1}{2}\right)^2=2y^2+\dfrac{1}{2}

But by the Trivial inequality, 2 y 2 + 1 2 1 2 2y^2+\dfrac{1}{2}\ge \dfrac{1}{2} so we are done.

Jake Lai
Jun 3, 2015

The minimum is achieved when x 2 = ( x + 1 ) 2 x^{2} = (x+1)^{2} - this is the equality case for AM-GM. (It can also be proven that the minimum is achieved using a multitude of other ways, including possibly Cauchy-Schwarz and even geometrically.)

Since x x + 1 x \neq x+1 , it must be that x = x + 1 -x = x+1 , ie x = 1 2 x = \boxed{-\frac{1}{2}} .

f(x) = x^2+ x^2+2x+1 = 2x^2+2x+1

Since coefficient of x^2 is positive, f(x) has a minimum turning point.

Turning point is at x= -2/2(2) = -1/2

So minimum is f(-1/2)

f(-1/2) = 1/2

So minimum value is 1/2

. .
May 8, 2021

x 2 + ( x + 1 ) 2 x ^ { 2 } + ( x + 1 ) ^ { 2 }

y = 2 x 2 + 2 x + 1 y = 2x ^ { 2 } + 2x + 1

y = 2 ( x + 1 2 ) 2 + 1 2 y = 2 ( x + \frac { 1 } { 2 } ) ^ { 2 } + \frac { 1 } { 2 }

Hence, 1 2 \frac { 1 } { 2 } .

Charles Chen
Aug 21, 2016

Sqrt(x^2+(x+1)^2) is the distance between (x,x) which is a point on line y=x and (0,-1). By graph the shortest distance is half the unit square's diagnol or 1/sqrt(2). Thus tye answer is 1/2

Nice graphical interpretation :)

Calvin Lin Staff - 4 years, 9 months ago
Vishnu C
May 31, 2015

The function is 2 x 2 + 2 x + 1 2x^2+2x+1 and its minimum is D 4 a = 1 2 -\frac D {4a} = \frac 1 2 .

Moderator note:

How do you know that it's a minimum value rather than a maximum value? And do you have proof that it must occur at D 4 a - \frac D{4a} ?

Can you tell what is D ???

Rama Devi - 6 years ago

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Discriminent

Srikishan Adepu - 6 years ago

Discriminant.

vishnu c - 6 years ago

Determinent

Srikishan Adepu - 6 years ago

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I think Discriminant is a better name

Rama Devi - 6 years ago

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@Rama Devi I am not wrong! It is called discriminant because it discriminates or decides whether the roots are real or not. I spelled it right.

vishnu c - 6 years ago

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@Vishnu C I am not telling that you are wrong.

Rama Devi - 6 years ago

In response to challenge master : a is positive. That means that it's a parabola opening upwards.

vishnu c - 6 years ago

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In response to challenge master : Going by the trend shown by your notes in my solution, I bet that you're eventually going to end up asking: "How is 1+1 = 2?"

vishnu c - 6 years ago

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Yet the simple things are difficult to solve. That's why you start out with axioms that make a sort of rough foundation for your domain.

And when you don't follow a certain axiom (Euclid's fifth for example), you may end up with a whole new foundation for your domain. Or you may end up in a mental health institution :)

vishnu c - 6 years ago

Nice conclusion, brah.

Jake Lai - 6 years ago
Hana Wehbi
May 10, 2016

To find a minimum or a maximum, we need the first derivative of the function, which is 4 x + 2 4x+2 , we set equal to 0 and solve for x x ; that is, 2 x + 2 ( x + 1 ) = 0 2x+2(x+1)=0 , we obtain x = ( 1 2 x=(\frac{-1}{2} ), then f ( 1 / 2 ) = 1 / 2 f(-1/2)=1/2

Samwan Tan
Jun 7, 2015

x^2+(x+1)^2=2x^2+2x+1

a[x+b/(2a)]^2+(4ac-b^2)/4a

minimum y-value=(4ac-b^2)/4a

which is [4(2)(1)-4]/8=1/2

Qonitah Jannah
Jun 6, 2015

Obvoiusly: the function which quadratic function, let f(x)=2x^2 +2x +1, f will be minimum when x = -1/2. Hence, f_min= 1/2

Jonathan Altman
Jun 3, 2015

Use calculus: take derivative of function; then set derivative equal to zero; finally, solve for x.

Tien Nguyen Le
Jun 3, 2015

Easy solution. 2X^2 + 2X +1 = 2X^2 + 2 (squar root of 2)X 1/(square root of 2) + 1/2 + 1/2 = ((square root of 2)X + 1/(square root of 2))^2 + 1/2 >= 1/2

Michael Mendrin
Jun 3, 2015

For symmetry reasons, it's gotta be in the middle between 1 -1 and 0 0 , and so we have ( 1 2 ) 2 + ( 1 2 ) 2 = 1 2 { \left( -\frac { 1 }{ 2 } \right) }^{ 2 }+{ \left( \frac { 1 }{ 2 } \right) }^{ 2 }=\frac { 1 }{ 2 }

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