# Thread: Simultaneous equations

1. ## Simultaneous equations

I'm having real grief with this problem

Emma draws the graph of the circle x2 + y2 = 25.
She is then told to draw the graph of the line x + y = –1 on the same set of axes.

Find the coordinates of both points where the line intersects the circle

2. Originally Posted by mathsdummy
I'm having real grief with this problem

Emma draws the graph of the circle x2 + y2 = 25.
She is then told to draw the graph of the line x + y = –1 on the same set of axes.

Find the coordinates of both points where the line intersects the circle
Not to difficult, all we have to do is think about it.
What info do we have?
Well we know that x + y = -1
now that means that y = -1 - x
sub this in to out x^2 + y ^2 =25 formula and we get
x^2 + (-1(x+1))^2 = 25
x^2 + (-1)^2 . (x + 1)^2 = 25
x^2 + (x + 1)^2 = 25
x^2 + x^2 +2x +1 -25 = 0
2x^2 + 2x -24 = 0
x^2 + x -12 = 0
By the cross method (x+4)(x-3) = 0
x = -4 and x = 3
Alternatively you can use quadratic fomula:
x = (-b +- ((b^2 - 4ac)^1/2))/2a
x = (-1 +- ( 1 + 48)^1/2)/2
x = (-1 +- 7)/2
x= -4 and 3
Therefore subing in these x values into x + y = -1:
-4 + y = -1
y = 3
Therefore one point is (-4,3)
3 + y = -1
y = -4
Therefore one point is (3,-4)
Therefore the 2 functions (actually i should say curves given that a circle isn't a function) meet at (-4,3) and (3, -4)

Edit: Could someone please tell me where I can find the code to put in symbols and equations, such as square root signs instead of ^1/2 , integral signs and dy/dx etc. Much appreciated

3. Originally Posted by behemoth100

Edit: Could someone please tell me where I can find the code to put in symbols and equations, such as square root signs instead of ^1/2 , integral signs and dy/dx etc. Much appreciated
are you familiar with LaTex? well, its not that difficult even if you aren't, i just started learning it myself about 2 days ago. See the http://www.mathhelpforum.com/math-he...-tutorial.html

4. Originally Posted by behemoth100
Edit: Could someone please tell me where I can find the code to put in symbols and equations, such as square root signs instead of ^1/2 , integral signs and dy/dx etc. Much appreciated
See the LaTeX tutorial here.

RonL

5. How do you do this? (By "this" I am refering to the caption in place of a url.)

Originally Posted by CaptainBlack

6. Originally Posted by ecMathGeek
How do you do this? (By "this" I am refering to the caption in place of a url.)
When creating a post, type the text you want (something like "here") select
the text, then press the button on the format bar (blue globe with a chain
link) this will give you a dialog box into which you can paste the link.

RonL