You are Here: Home >< Maths

# circle geometry - translation - C1 watch

1. question: write down the equation of the circle which is obtained by applying the given translation to the given circle:

-1
2

(that's supposed to be the vector idk how to do extended square brackets)

the given circle: x2 + y2 - 4x + 2y = 4

what I did was group the x & y's and complete the square to get (x-2)2 + (y+1)2 = 9 and then applied the translation to get an answer of: (x -1)2 + (y-3)2 = 9

but in the answers it says: x2 + y2 - 2x - 2y = 7

even if I expand out the answer that I got, I don't get the same - can someone please tell me what I've wrong

and also how to directly apply a translation to an equation in same form as the one in the question without having to complete the square
2. It's the y part you have done incorrectly.
3. (Original post by batoot)
question: write down the equation of the circle which is obtained by applying the given translation to the given circle:

-1
2

(that's supposed to be the vector idk how to do extended square brackets)

the given circle: x2 + y2 - 4x + 2y = 4

what I did was group the x & y's and complete the square to get (x-2)2 + (y+1)2 = 9 and then applied the translation to get an answer of: (x -1)2 + (y-3)2 = 9

but in the answers it says: x2 + y2 - 2x - 2y = 7

even if I expand out the answer that I got, I don't get the same - can someone please tell me what I've wrong

and also how to directly apply a translation to an equation in same form as the one in the question without having to complete the square
You should always complete the square for equation of a circle - that is the standard form it is notmally in.
4. (Original post by Ano123)
It's the y part you have done incorrectly.
i don't get where though, I just checked through my working again
5. (Original post by batoot)
i don't get where though, I just checked through my working again
The y-coordinate of the centre is (, -1) - if you shift this up by two units, you should get it (, +1), so your new equation should be...?
6. (Original post by Zacken)
The y-coordinate of the centre is (, -1) - if you shift this up by two units, you should get it (, +1), so your new equation should be...?
ooooooooh -1
so it should be
(x -1)2 + (y-1)2 = 9

I can't believe I didn't see that -.- I always make stupid mistakes, thank you
7. (Original post by batoot)
ooooooooh -1
so it should be
(x -1)2 + (y-1)2 = 9

I can't believe I didn't see that -.- I always make stupid mistakes, thank you
No problemo. :-)

### Related university courses

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: February 16, 2016
The home of Results and Clearing

### 2,170

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Sheffield Hallam University
Tue, 21 Aug '18
2. Bournemouth University
Wed, 22 Aug '18
3. University of Buckingham
Thu, 23 Aug '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