# Thread: Cubic Equation

1. ## Cubic Equation

Solve the cubic equation $\displaystyle 3x^3-x^2-8x-4=0$. Hence solve the equation $\displaystyle 3(y+1)^3-(y+1)^2=8y+12$

I have already solved the cubic equation however the 2nd part makes no sense to me.

Answers to cubic equation is $\displaystyle (x+1)(3x+2)(x-2)$

MY GUESS is that the solution to solving part 2 is changing the y in 8y+12 to (y+1) so as to substitute y+1 = x

2. Originally Posted by Punch
Solve the cubic equation $\displaystyle 3x^3-x^2-8x-4=0$. Hence solve the equation $\displaystyle 3(y+1)^3-(y+1)^2=8y+12$

I have already solved the cubic equation however the 2nd part makes no sense to me.

Answers to cubic equation is $\displaystyle (x+1)(3x+2)(x-2)$

MY GUESS is that the solution to solving part 2 is changing the y in 8y+12 to (y+1) so as to substitute y+1 = x
$\displaystyle 3(y+1)^3-(y+1)^2=8y+12 \Rightarrow 3(y+1)^3-(y+1)^2 - 8(y+1) + 8 - 12 = 0$ ....

Then y + 1 = x ....

3. sorry but how did you get the y+1 for the 8 and an extra 8 popping out?

can you show me a more detailed working as i am not very expert in maths

4. Originally Posted by Punch
sorry but how did you get the y+1 for the 8 and an extra 8 popping out?

can you show me a more detailed working as i am not very expert in maths
You're expected to know that 8(y + 1) = 8y + 8 and therefore 8y = 8(y + 1) - 8.

And it should be clear why I wrote 8y as 8(y + 1) - 8.

By the way, I see from the time stamps that you spent exactly 2 minutes thinking about my reply (even less if the time taken to type your reply is taken into account). You're expected to put in much more effort than 2 minutes when trying to understand the help you get given.

5. Originally Posted by mr fantastic
By the way, I see from the time stamps that you spent exactly 2 minutes thinking about my reply (even less if the time taken to type your reply is taken into account). You're expected to put in much more effort than 2 minutes when trying to understand the help you get given.