Express x^2 - 10x in the form (x+p)^2 + q

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Homework Help Overview

The problem involves expressing the quadratic expression x^2 - 10x in the form (x+p)^2 + q. Participants are tasked with identifying the values of p and q.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Some participants attempt to expand the expression (x+p)^2 + q and compare coefficients to find p and q. Others suggest completing the square as a more comfortable method. There are questions about the correctness of the steps taken and the interpretation of the expression versus an equation.

Discussion Status

The discussion includes various attempts to complete the square and clarify the relationship between the terms. Some participants express uncertainty about their calculations, while others provide insights into the process of finding p and q. There is no explicit consensus on the values, but multiple interpretations and methods are being explored.

Contextual Notes

Participants note the distinction between working with an expression versus an equation, and there are discussions about the implications of manipulating terms in the context of completing the square.

Haroldingo
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Homework Statement



Express x^2 - 10x in the form (x+p)^2 + q

State the value of P and Q

The Attempt at a Solution



I don't know! I don't get it because when I times out the brackets p will always be a number, and there are no numbers that aren't multiplied by x in x squared minus 10 x. Gragh?
 
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Expand this out: (x+p)^2 + q

Compare the coefficients of this expression to x^2 - 10x (the constant term here is zero, i.e. the expression can be written x^2 - 10x + 0). What equations can you set up to define p and q?
 
Ok so I complete the square because I'm more comfortable with that:

x^2 - 10x + 0 = 0

x^2 - 10x = 0

x^2 - 5^2 = 5^2

(x-5)^2 - 25

this would mean p = 5 and q = -25?

This doesn't seem right? Have I gone wrong somewhere?
 
Last edited:
Haroldingo said:

Homework Statement



Express x^2 - 10x in the form (x+p)^2 + q

State the value of P and Q

The Attempt at a Solution



I don't know! I don't get it because when I times out the brackets p will always be a number, and there are no numbers that aren't multiplied by x in x squared minus 10 x. Gragh?

Haroldingo said:
Ok so I complete the square because I'm more comfortable with that:

x^2 - 10x + 0 = 0

x^2 - 10x = 0

x^2 - 5^2 = 5^2

(x-5)^2 - 25

this would mean p = 5 and q = -25?

This doesn't seem right? Have I gone wrong somewhere?

It should be right as you wrote:
x2-10x <=> (x+p)(x+p) + q <=> x2+2xp+p2+q.
x2-5x-5x+25-25 <=> x2-10x

p=5, q=-25
 
p=-5 q=-25

I find it quicker to half the x term... so there's your p straight away -5

and then sqaure p so -5^2 = 25 and you need +0 so its -25...

another example is
x^2 - 6x + 30

so again half -6 is p= -3
then again square -3 = 9
and i need 30 so 9+21 q =21
 
Cheers all, got the marks! :)
 
Some quibbles:
Haroldingo said:
Ok so I complete the square because I'm more comfortable with that:
x^2 - 10x + 0 = 0
You're not working with an equation - just an expression.
Haroldingo said:
x^2 - 10x = 0

x^2 - 5^2 = 5^2
Didn't notice earlier, but this isn't correct.
Where did the -10x term go? And how is it that you can add -5^2 to one side of an equation, but add +5^2 to the other. The answer is, you can't do this.
Haroldingo said:
(x-5)^2 - 25

this would mean p = 5 and q = -25?
Starting at the beginning, you have
x2 - 10x
= x2 - 10x + 25 - 25
= (x - 5)2 - 25
= (x + (-5))2 + (-25)
I leave it for you to figure out what p and q are.
Haroldingo said:
This doesn't seem right? Have I gone wrong somewhere?

Gliese123 said:
It should be right as you wrote:
x2-10x <=> (x+p)(x+p) + q <=> x2+2xp+p2+q.
x2-5x-5x+25-25 <=> x2-10x

p=5, q=-25
Gliese123, Do not connect expressions with an equivalent sign. Expressions that are equal should be connected with =. Statements such as equations or inequalities can in some cases be connected with the equivalent symbol, <=>.
 

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