Proving Inequality: x^2+2xy+3y^2+2x+6y+4 >= 1 for All Real Values of x and y

In summary, the conversation discusses how to prove that the expression x^2+2xy+3y^2+2x+6y+4 is greater than or equal to 1 for all real values of x and y. One suggestion is to treat the expression as a function and find the minimum point, while another suggests considering the expression as a pair of parallel lines. The concept of a function of two variables is also briefly discussed.
  • #1
siddharthmishra19
27
0
For all real values of x and y prove that
x^2+2xy+3y^2+2x+6y+4 >= 1

I am trying to express the left side as a sum of sqares but always come short. Help will be appreciated.
 
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  • #2
You are going to have trouble doing it that way. Just treat the problem as the problem of finding the minimum of the function on the LHS of your inequality. Find the point where the two partial derivatives vanish and verify the that point is a minimum and that it's >= 1.
 
  • #3
I want to float another option. In this equation, [tex]abc+2fgh-af^2-bg^2-ch^2[/tex] is not 0. Therefore this curve represents a pair of straight lines. Geometrically this means that the pair of lines must always remain above the line y=1. The only way this is possible is if the pair of lines is parallel. If you find the equations of the two lines, then you should find that they lie above y=1.
 
  • #4
chaoseverlasting said:
I want to float another option. In this equation, [tex]abc+2fgh-af^2-bg^2-ch^2[/tex] is not 0. Therefore this curve represents a pair of straight lines. Geometrically this means that the pair of lines must always remain above the line y=1. The only way this is possible is if the pair of lines is parallel. If you find the equations of the two lines, then you should find that they lie above y=1.

In what sense is a function of two variables a 'curve'? A level surface would be a curve - but which one? I don't get it.
 
  • #5
Sorry, I meant a conic section, or a pair of straight lines in the xy plane of the type [tex]ax^2+2hxy+by^2+2gx+2fy+c=0[/tex]. Here though, even if delta is not 0, it doesn't seem to be a straight line (the two constant terms are coming out to be complex). If you add 2 to both sides of the inequality, then this equation represents an ellipse with its axis rotated. So, the lowest point must lie above the line y=3.
 
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FAQ: Proving Inequality: x^2+2xy+3y^2+2x+6y+4 >= 1 for All Real Values of x and y

What is an "inequality proof"?

An inequality proof is a mathematical technique used to demonstrate that one quantity is either greater or less than another quantity. This is typically done by manipulating equations or using logical arguments.

Why are inequality proofs important in science?

Inequality proofs are important in science because they allow us to make comparisons and draw conclusions about different quantities. This is especially useful in fields like economics, physics, and biology where understanding the relationships between different variables is crucial.

How do you construct an inequality proof?

An inequality proof involves breaking down the problem into smaller, more manageable parts and then using logical arguments or mathematical equations to show how one quantity is greater or less than another. It is important to clearly state your assumptions and show all of your steps in the proof.

Can you provide an example of an inequality proof?

Sure! Here is a simple example: Prove that for any positive real numbers a and b, a + b > 2√(ab). First, we can square both sides to get (a + b)^2 > 4ab. Then, we can expand the left side to get a^2 + 2ab + b^2 > 4ab. Next, we can subtract 4ab from both sides to get a^2 - 2ab + b^2 > 0. This can be factored into (a - b)^2 > 0, which is always true for positive real numbers. Therefore, the original inequality is proven.

Are there any common mistakes to avoid when constructing an inequality proof?

One common mistake is assuming that the inequality is true for all values of the variables, when in fact it may only be true for a certain range of values. It is important to clearly state any restrictions or assumptions in the proof. Additionally, it is important to check for any mathematical errors or logical fallacies in the proof before declaring it complete.

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