Factorising Quadratics

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I don't understand Step 4. 9-2=7 so shouldn't the brackets be: (3x+2)(x-3)? I think this because don't you just put the signs in where they come from your equation? What I mean is 9 was first so the first bracket should have a + sign.

Reply 1

Dalek1099

21

Original post by UrGrandpap

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I don't understand Step 4. 9-2=7 so shouldn't the brackets be: (3x+2)(x-3)? I think this because don't you just put the signs in where they come from your equation? What I mean is 9 was first so the first bracket should have a + sign.

No because the x term in (3x+2)(x-3) is 3x*-3+2*x=-9x+2x=-7x not 7x. You should expand the brackets out to work out what the right signs are if you are confused.

Reply 2

Harry-Pikesley

11

No, if you were to expand that, you would get 3 * 3x and -2 * x, giving 9x - 2x, which is 7x

Reply 3

UrGrandpap

OP

14

Original post by Dalek1099

No because the x term in (3x+2)(x-3) is 3x*-3+2*x=-9x+2x=-7x not 7x. You should expand the brackets out to work out what the right signs are if you are confused.

I understand what you're saying but on step 4 it says fill in the signs so 9 and 2 add/subtract to give +7. +9-2=+7 so I'm not sure why it's the opposite.

Original post by Harry-Pikesley

No, if you were to expand that, you would get 3 * 3x and -2 * x, giving 9x - 2x, which is 7x

Reply 4

Dalek1099

21

Original post by UrGrandpap

I understand what you're saying but on step 4 it says fill in the signs so 9 and 2 add/subtract to give +7. +9-2=+7 so I'm not sure why it's the opposite.

Exactly so you have to consider both ways of filling in the signs and check which one gives the right answer as I demonstrated in my previous post your answer gives -9+2=-7 so thats not the correct way of filling it the signs so they must be the opposite way around.

Reply 5

UrGrandpap

OP

14

Original post by Dalek1099

Exactly so you have to consider both ways of filling in the signs and check which one gives the right answer as I demonstrated in my previous post your answer gives -9+2=-7 so thats not the correct way of filling it the signs so they must be the opposite way around.

I think I see what you mean. Tha ks for the help!

Factorising Quadratics

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