Urgent C1 help needed

QP
http://www.tomred.org/uploads/7/7/8/3/778329/c1_qp_mock.pdf
MS
http://www.tomred.org/uploads/7/7/8/3/778329/c1_ms_mock.pdf

Look at 5b. Here's my working out
x^2+8x-1=0
(x+4)^2-16-1=0
(x+4)^2-17=0
(x+4)^2=17
x=-4±√(17)

But in the MS they've got a 2 in front of the the √(17). Can somebody please explain? I don't understand why you can't use completing the square an my exam is soon :frown:

Thank you

Reply 1

TenOfThem

18

Original post by krisshP

QP
http://www.tomred.org/uploads/7/7/8/3/778329/c1_qp_mock.pdf
MS
http://www.tomred.org/uploads/7/7/8/3/778329/c1_ms_mock.pdf

Look at 5b. Here's my working out
x^2+8x-1=0
(x+4)^2-16-1=0
(x+4)^2-17=0
(x+4)^2=17
x=-4±√(17)

But in the MS they've got a 2 in front of the the √(17). Can somebody please explain? I don't understand why you can't use completing the square an my exam is soon :frown:

Thank you

You can
You are right
The MS is wrong

Reply 2

justinawe

19

Original post by krisshP

QP
http://www.tomred.org/uploads/7/7/8/3/778329/c1_qp_mock.pdf
MS
http://www.tomred.org/uploads/7/7/8/3/778329/c1_ms_mock.pdf

Look at 5b. Here's my working out
x^2+8x-1=0
(x+4)^2-16-1=0
(x+4)^2-17=0
(x+4)^2=17
x=-4±√(17)

But in the MS they've got a 2 in front of the the √(17). Can somebody please explain? I don't understand why you can't use completing the square an my exam is soon :frown:

Thank you

You are correct, the markscheme is wrong.

Reply 3

krisshP

OP

14

Original post by TenOfThem

You can
You are right
The MS is wrong

Original post by justinawe

You are correct, the markscheme is wrong.

Thanks so much. I was getting so worried and sacred that I can't even solve a quadratic when my C1 exam is tomorrow :eek:

Thanks!

Reply 4

StalkeR47

5

Original post by krisshP

QP
http://www.tomred.org/uploads/7/7/8/3/778329/c1_qp_mock.pdf
MS
http://www.tomred.org/uploads/7/7/8/3/778329/c1_ms_mock.pdf

Look at 5b. Here's my working out
x^2+8x-1=0
(x+4)^2-16-1=0
(x+4)^2-17=0
(x+4)^2=17
x=-4±√(17)

But in the MS they've got a 2 in front of the the √(17). Can somebody please explain? I don't understand why you can't use completing the square an my exam is soon :frown:

Thank you

Hi, I did this question myself. And I got exactly what you got. Mark scheme is wrong!

Reply 5

ell_

I got a 2 in front of the 17 :/
You use the equation from part a:
x^2+8x-1=0
Then use the quadratic formula
a=1
b=8
c=-1
This gives you: -8 +or- root 68 / 2
Which then gives -8 +or- 4root17/2 cancelling to give -4+or- 2root17
Sorry I didn't know how to get the square root symbol :L

So, I recommend trying it with the quadratic fromula because I don't think MS is wrong :smile:

(edited 10 years ago)

Reply 6

justinawe

19

Original post by ell_

I got a 2 in front of the 17 :/
You use the equation from part a:
x^2+8x-1=0
Then use the quadratic formula
a=1
b=8
c=-1
This gives you: -8 +or- root 68 / 2
Which then gives -8 +or- 4root17/2 cancelling to give -4+or- 2root17
Sorry I didn't know how to get the square root symbol :L