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# Using the Quadratic Formula...just a matter of plugging numbers into a formula?

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. So how does the quadratic formula really work? I'm a little confused, I'm pretty sure I've muddled up what my teacher told me.

anyhow, I got the question "3x^2+4x-12=0"

I ended up with the answer -8.43 & -2.77
Apparently these are wrong

Apparently I suck at algebra

Can anyone help me find where I went wrong?
2. The funny thing is, it is just plugging in numbers however lots of people trip up when using it.

1. Write down the formula

2. Write down what a, b and c is

3. Write down the formula with the substituted values

4. Calculate

I think where you have gone wrong is that you haven't put negatives in brackets. All negative numbers should be in brackets. A point to note as well:

Lets say b = -5

It isn't (--5) and it isn't --(5). It is -(-5)

However, lets say b = 5

It isn't (-5), it is -(5) because it is negative b, not negative 5 (I know that sounds pedantic but trust me)
x = 1.44 or x = -2.77 correct???

EDIT: it is very easy to go wrong here. However if you do those steps then you will be less likely to make a mistake.
3. (Original post by BTAnonymous)
The funny thing is, it is just plugging in numbers however lots of people trip up when using it.

1. Write down the formula

2. Write down what a, b and c is

3. Write down the formula with the substituted values

4. Calculate

I think where you have gone wrong is that you haven't put negatives in brackets. All negative numbers should be in brackets. A point to note as well:

Lets say b = -5

It isn't (--5) and it isn't --(5). It is -(-5)

However, lets say b = 5

It isn't (-5), it is -(5) because it is negative b, not negative 5 (I know that sounds pedantic but trust me)
x = 1.44 or x = -2.77 correct???

EDIT: it is very easy to go wrong here. However if you do those steps then you will be less likely to make a mistake.

Thank you very much for this!

I see what you mean by the negative signs - very pedantic, yes, but I'll remember it for next time. This is very helpful actually, so thanks a lot.

:-)
4. Brackets are your friends especially when substituting in a number into a large equation i.e. quadratic formula

Always see if you can factorise or complete the square first - Generally quicker and less likely to make a mistake.
5. (Original post by ZiggyStarDust_)
Thank you very much for this!

I see what you mean by the negative signs - very pedantic, yes, but I'll remember it for next time. This is very helpful actually, so thanks a lot.

:-)
You're welcome

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