# Thread: [SOLVED] Equation of 2 graphs

1. ## [SOLVED] Equation of 2 graphs

Hi everyone I have a maths question as follows:

Plot the graphs of y=x^2+x-1 and y=x+1 as accurately as possible on the same axis using value of x from -4 to +4

Write down an equation for the values of x where the graphs meet and solve this equation. Check and show the solution on the graph.

OK so I have drawn it by hand but to illustrate I plotted it in excel for you all to see here:

Can anyone tell me how to find out the equation I need to answer this question please?

Thanks for your time - James

2. ## set them equal to each other

$\displaystyle y=x^2+x-1$ and $\displaystyle y=x+1$

since these equations both =y then set them equal to each other

so $\displaystyle x^2 + x -1 = x+1$ then solve for $\displaystyle x$ to get the $\displaystyle x$ values

thus

$\displaystyle x^2 = 2 \rightarrow x = \pm\sqrt{2}$

3. Originally Posted by bigwave
$\displaystyle y=x^2+x-1$ and $\displaystyle y=x+1$

since these equations both =y then set them equal to each other

so $\displaystyle x^2 + x -1 = x+1$ then solve for $\displaystyle x$ to get the $\displaystyle x$ values

thus

$\displaystyle x^2 = 2 \rightarrow x = \pm\sqrt{2}$
So how did you get to x^2=2 ?

+1 each side then subtract x's ?

4. Originally Posted by JamesC1705
So how did you get to x^2=2 ?

+1 each side then subtract x's ?
$\displaystyle x^2 + x -1 = x +1$

add $\displaystyle 1$ to both sides then

$\displaystyle x^2 + x = x + 2$

subtract $\displaystyle x$ from both sides then

$\displaystyle x^2 = 2$

thus

$\displaystyle x = \pm\sqrt{2}$

5. Ok thanks for all your help

Just have to write it all out now

Next up is an equation of a straight line graph but I think I can research and figure out the answer but you never know haha watch this space