You are Here: Home >< Maths

# need help with difference of two squares please watch

1. i worked this question out before but now for some reason i dont know how to do it now
the question is solve 4x^2-8=0 using differnce of two squares

there's also another one, solve 2x^2-8x-11=0 using the completed square method
HELP!
thank you
2. 2x^2 - 8x - 11 = 0

deal with the first two terms only

2x^2 - 8x = 0

factorise out the 2

2(x^2 - 4x) = 0

square the bracket

2((x - 2)^2) = 0

subtract the number in the bracket and square it

2((x - 2)^2 - 2^2) = 0

bring down the constant

(x - 2)^2 - 8 - 11 = 0

(x - 2)^2 - 19 = 0
3. (Original post by LazyWorseThanInfidel)
2x^2 - 8x - 11 = 0

deal with the first two terms only

2x^2 - 8x = 0

factorise out the 2

2(x^2 - 4x) = 0

square the bracket

2((x - 2)^2) = 0

subtract the number in the bracket and square it

2((x - 2)^2 - 2^2) = 0

bring down the constant

(x - 2)^2 - 8 - 11 = 0

(x - 2)^2 - 19 = 0
All of these bolded lines are wrong. I can see what you're doing, and it's the right method, but please don't mislead people into thinking that these things are actually equal to 0. What you should be doing is:

2x^2 - 8x - 11 = 0
(2x^2 - 8x) - 11 = 0
2(x^2 - 4x) - 11 = 0
2[(x - 2)^2 - 2^2] - 11 = 0

...and for some reason, in the two underlined lines, you dropped the factor of 2.
4. sorry il have to step it up a league well spotted

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: October 12, 2009
Today on TSR

### The TSR A-level options discussion thread

Choosing A-levels is hard... we're here to help

### University open days

• Heriot-Watt University
School of Textiles and Design Undergraduate
Fri, 16 Nov '18
• University of Roehampton
Sat, 17 Nov '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 17 Nov '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams