How Can I Simplify a Quadratic Equation with Rationals?

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Discussion Overview

The discussion revolves around the process of rewriting a quadratic equation in the form a(x - h)^2 + k, specifically focusing on the equation y = x^2 + 3x + 5/2. Participants explore methods for completing the square, particularly when rational numbers are involved.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks guidance on rewriting the quadratic equation with rationals, expressing uncertainty about the process.
  • Another participant suggests completing the square and mentions the option of multiplying through by two to eliminate the rationals, with a caution to divide back at the end.
  • A participant asserts that the method for completing the square remains the same regardless of the presence of rationals.
  • Further elaboration is provided on the requirement for c to equal (b/2)^2 to achieve a complete square, with specific calculations presented to adjust the constant term in the equation.
  • One participant questions the connection between the proposed steps and the original question, seeking clarification on the equivalence of the expressions presented.
  • Another participant confirms the equivalence of the completed square form and the original quadratic, suggesting that rearrangement can reveal the desired form.

Areas of Agreement / Disagreement

There is no consensus on the clarity of the steps provided, as some participants express confusion regarding the connections between the equations. Multiple viewpoints on the method of completing the square are presented, indicating that the discussion remains unresolved in terms of clarity and understanding.

Contextual Notes

Participants express varying levels of understanding regarding the completion of the square, particularly with rational numbers, and the discussion includes specific calculations that may depend on individual interpretations of the process.

Caldus
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Trying to rewrite a quadratic equation in the form a(x - h)^2 + k. The equation I'm trying to rewrite is:

y = x^2 + 3x + 5/2

Not looking for an answer, just looking for how to do this (I don't know how to do it if it has rationals in it). Thank you.
 
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Complete the square.
x2+3x +k=(x+3/2)2
What is k?

Also, you can multiply through by two to get rid of the rationals, but don't forget to divide it out at the end.
 
Last edited:
IOW you do it the same as if there weren't rationals.
 
Originally posted by StephenPrivitera
x2+3x +k=(x+3/2)2
I am sorry, but, how are those two equal ? and how are they connected to the original question ?

To convert a quadratic to the form (a(x-h)^2 + k) you must (as StephenPrivitera said) complete the square.
If you have a quadratic on the form of :
ax^2 + bx + c
Then, it is a complete square if c=(b/2)^2
So, to turn any quadratic to a complete square you need to make (c) in it equal to ((b/2)^2)
In your case, (b/2)^2 = (3/2)^2 = 9/4
To turn 5/2 into 9/4, you will need to add ((9/4)-(5/2)=(9/4)-(10/4)=-(1/4)) to it. But if you add any number to the quadratic you will actually change its value. So, to maintain the value, you will subtract the same number again, therefore leaving the qudratic unchanged (adding and subtracting the same number is like adding 0, it does nothing to the quadratic).
Here you go:
y = x^2 + 3x + 5/2
y = x^2 + 3x + 5/2 + 0
y = x^2 + 3x + 5/2 - 1/4 + 1/4
y = x^2 + 3x + (5/2 - 1/4) + 1/4
y = x^2 + 3x + (10/4 - 1/4) + 1/4
y = x^2 + 3x + 9/4 + 1/4
y = (x^2 + 3x + 9/4) + 1/4
y = ((x + 3/2)*(x + 3/2)) + 1/4
y = (x + 3/2)^2 + 1/4

Which is on the form that you asked for :smile:.
 
I am sorry, but, how are those two equal ? and how are they connected to the original question ?


Notice that you proved:

x^2 + 3x + 5/2 = (x + 3/2)^2 + 1/4

Do a little rearrangement and you'll see that's (essentially) of the form

x^2+3x +k=(x+3/2)^2
 

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