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# How does Corbett get this answer - Equation? watch

1. How did Corbett get this answer?

Solve x^2=51+14x

His solves it by

X^2 - 14x-51=0

(x-17) (x+3)

x=17 or -3

How does he get the 17, and I presume it becomes -3 because its a +-=-?

2. (Original post by Purdy7)
How did Corbett get this answer?

Solve x^2=51+14x

His solves it by

X^2 - 14x-51=0

(x-17) (x+3)

x=17 or -3

How does he get the 17, and I presume it becomes -3 because its a +-=-?

If (x - 17)(x + 3) = 0

then

x - 17 = 0 or x + 3 = 0

Solving these little equations gives you x = 17 or x = - 3.
3. But how did he get the 17 in the first place neither is the 3? It's not even in the original equation.
4. (Original post by Purdy7)
But how did he get the 17 in the first place neither is the 3? It's not even in the original equation.
If you expand the brackets you will see that the factorisation is correct.

(x-17)(x+3)= x^2 +3x-17x-51=x^2-14x-51.

I would recommend some practice expanding brackets and then some practice factorising. You might need some advice before you try the factorising. When you're ready, this page http://burymathstutor.co.uk/quads10.html will serve up randomly generated questions (and solutions) for you to answer.
5. I'm fine expanding brackets. They aren't in the original question which I thought was a quadratic expression therefore would have solved it that route and not factorisation. .

Personally I think it's his bad for not explaining the full answer to the question, which he normally does.
6. (Original post by Purdy7)
I'm fine expanding brackets. They aren't in the original question which I thought was a quadratic expression therefore would have solved it that route and not factorisation. .

Personally I think it's his bad for not explaining the full answer to the question, which he normally does.

It's a quadratic equation. When they are easy to factorise, factorising is the quickest way to solve them.
7. (Original post by Purdy7)
How did Corbett get this answer?

Solve x^2=51+14x

His solves it by

X^2 - 14x-51=0

(x-17) (x+3)

x=17 or -3

How does he get the 17, and I presume it becomes -3 because its a +-=-?

Factorise -51. -3 and 17, or 3 and -17.

Which pair of factors when added produce the desired -14

For any quadratic of the form x^2 + bx + c = 0

factors will be (x + m)(x +n) with solutions x= -m and x= -n

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