# Mathematics questions

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## 30 Replies - 3040 Views - Last Post: 21 May 2010 - 11:26 AM

### #1 FrozenSnake

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# Mathematics questions

Posted 10 May 2010 - 12:51 PM

Again I am stuck in math this time it's this task:
Simplify the following expression as far as possible:
(x - 4)² - 16

I don't understand how to solve this one. The answer is NOT x = 20
I have masked the answer in case someone wish to give it a try themselves before trying to help me

I tried the x² + px + q = 0 formula but that became not even near the answer.

Anyone that can describe this problem in a step by step fashion?

Note: I will get stuck again, and instead of making a new thread I will reuse this one. With the newest question ON TOP the older questions, marked with date to make it easy to see where the current problem ends.

This post has been edited by FrozenSnake: 10 May 2010 - 12:53 PM

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## Replies To: Mathematics questions

### #2 macosxnerd101

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## Re: Mathematics questions

Posted 10 May 2010 - 12:55 PM

When you evaluate the (x-4)^2, you get x^2 - 8x + 16. The positive and negative 16s then cancel out, leaving x^2 - 8x.

### #3 FrozenSnake

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## Re: Mathematics questions

Posted 10 May 2010 - 01:18 PM

Is it that simple? No need for the pq-formula? ;/ Everyone I asked said "X = 20" and the rest said "use pq-formula".

### #4 macosxnerd101

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## Re: Mathematics questions

Posted 10 May 2010 - 01:20 PM

The pq-formula you are referencing is the standard form of a quadratic equation. We are foiling the (x-4)^2 to get standard form, then subtracting the 16, still essentially leaving us in standard form.

### #5 FrozenSnake

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## Re: Mathematics questions

Posted 10 May 2010 - 01:23 PM

Beh, it was so easy all my friends didn't see it Thanks

### #6 NeoTifa

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## Re: Mathematics questions

Posted 14 May 2010 - 09:11 AM

Always try foiling first. If there is no absolute way to simplify, then use the quadratic equation.

### #7 Nikhil_07n

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## Re: Mathematics questions

Posted 15 May 2010 - 06:44 AM

Always use BEDMAS rule. Follow the sequence and nothing(NOTHING!) can go wrong.

B = Brackets
E = Exponentiation
D = Division
M = Multiplication
S = Subtraction

(Ordered by Decreasing priority)

Also, Never assume anything in math without solving the question first.

### #8 jjl

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## Re: Mathematics questions

Posted 16 May 2010 - 02:23 PM

well did you try plugging in 20?

(20-4)^2 = 16 (false)
16^2 != 16 (which is obviously is false)

foil:
x^2 - 8x = 0

simplify
(x-4)^2 = 16

goes to

(x^2-4x-4x+16) = 16

=> x^2-8x = 0

=>x(x-8)

i hate algebra, i guess thats why i liked calculus lol

This post has been edited by ImaSexy: 16 May 2010 - 03:42 PM

### #9 Swifteater

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## Re: Mathematics questions

Posted 16 May 2010 - 02:49 PM

I'm feeling like an idiot here as you've all done something different to my understanding of the task.

"Simplify the following expression as far as possible:
(x - 4)² - 16
"

To me means all you do is Square x giving x². Then Squaring -4, giving 16. Which gives you x² + 16 - 16 which simplified is just x². There is no solving involved as it is not an equation to solve (no equals). Can anyone show me how macosxnerd101 got -8x and how ImaSexy turned it into an equation and plugged in numbers in place of x when x is equal to anything since this expression is equal to nothing (not zero, it has not been given an equals so therefore x is impossible to determine)?

Sorry if I'm a bit of an idiot but that's just me.

Swifteater

### #10 jjl

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## Re: Mathematics questions

Posted 16 May 2010 - 03:24 PM

Quote

To me means all you do is Square x giving x². Then Squaring -4, giving 16. Which gives you x² + 16 - 16 which simplified is just x²

no you cannot just square each term

is this
(2^2+4^2) = (2+4)^2 ??

not its not, the first equals 20 and the second equals 36

you must FOIL (FIRST OUTER INNER LAST) the expression

(x-4)^2 = 16

goes to

(x^2-4x-4x+16) = 16

=> x^2-8x = 0

=>x(x-8)

This post has been edited by ImaSexy: 16 May 2010 - 03:40 PM

### #11 Nikhil_07n

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## Re: Mathematics questions

Posted 16 May 2010 - 03:30 PM

ImaSexy, on 17 May 2010 - 02:53 AM, said:

well did you try plugging in 20?

(20-4)^2 = 16 (false)
16^2 != 16 (which is obviously is false)

..
..

i hate algebra, i guess thats why i liked calculus lol

You're wrong! Because:
1)The equation simplifies to x^2 - 8x and that's the end.
2)The 'wrong' equation you've solved has a negative determinant, i.e., it has imaginary roots.

You're getting the correct answer JUST because somehow it's in your mind. 8+sqrt((-8)^2-4(1)(32))/(2*1) & 8-sqrt((-8)^2-4(1)(32))/(2*1) never give any value of x because the quantity inside the sqrt(the determinant) is negative.

Swifteater, on 17 May 2010 - 03:19 AM, said:

I'm feeling like an idiot here as you've all done something different to my understanding of the task.
..
..
Swifteater

You're solving (x-4)^2 to (x)^2 + (4)^2, which is wrong.

(x-4)^2 = (x)^2 + (4)^2 - 2(x)(4)

What macosxnerd101 has done is absolutely correct.

This post has been edited by Nikhil_07n: 16 May 2010 - 03:32 PM

### #12 jjl

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## Re: Mathematics questions

Posted 16 May 2010 - 03:37 PM

Quote

You're solving (x-4)^2 to (x)^2 + (4)^2, which is wrong.

(x-4)^2 = (x)^2 + (4)^2 - 2(x)(4)

What macosxnerd101 has done is absolutely correct.

No i did it correctly my friend, I changed my post right after i posted it.

EDIT: WTF, i guess my post didn't save when i corrected it. Ill change it know, ill correct your rep on your next post

This post has been edited by ImaSexy: 16 May 2010 - 03:39 PM

### #13 Swifteater

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## Re: Mathematics questions

Posted 16 May 2010 - 03:39 PM

Sorry, I'm an idiot. Just realised where the -8x is coming from.
(x-4)² = (x-4)(x-4)...

I understand now... talk about a dumbass moment.

Swifteater

### #14 jjl

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## Re: Mathematics questions

Posted 16 May 2010 - 03:43 PM

lol its ok, i just had one too.

### #15 Nikhil_07n

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## Re: Mathematics questions

Posted 16 May 2010 - 03:44 PM

ImaSexy, on 17 May 2010 - 04:07 AM, said:

Quote

You're solving (x-4)^2 to (x)^2 + (4)^2, which is wrong.

(x-4)^2 = (x)^2 + (4)^2 - 2(x)(4)

What macosxnerd101 has done is absolutely correct.

No i did it correctly my friend, I changed my post right after i posted it.

EDIT: WTF, i guess my post didn't save when i corrected it. Ill change it know, ill correct your rep on your next post

It's Okay!