1. ## tangent line equation

Hey, im having some problems with this

Find the equation of the line tangent to the curve y=2x^4-x^3 at the point (-1, 3). Any help appreciated.

i plugged the ordered pair into the genreal equation of a line, and now im stuck. What would the dy/dx for y=2x^4-x^3 be?

any help appreciated, thanks

2. You just wanna find the derivative of the original function...

So if $\displaystyle y=2x^4-x^3$ then $\displaystyle y'=8x^3-3x^2$

Use the x value you were given in the point (-1,3) and you will get that

$\displaystyle y'(-1)=-11$

So, now that you have a slope and a point, you can find the slop of the line by using the formula $\displaystyle (y-a)=m(x-b)$ So you get the equation of the line to be $\displaystyle y=-11x+32$

3. wait, shouldnt the final answer be y=-11x-8

4. Actually...on second thought...I mixed up my numbers a little bit there

It should be $\displaystyle y=-11x-14$

5. Originally Posted by Kalter Tod
Actually...on second thought...I mixed up my numbers a little bit there

It should be $\displaystyle y=-11x-14$
when i plug this into my calculator it doesnt even touch the parabola. isnt the answer supposed to be y=-11x-8, thats what i got when I solved and thats what the calc is saying for a tangent line to the parabola at X=-1

6. Haha yeah it is...Apparently, I can't add numbers.

I did it all in my head, so sorry for the math mistakes haha

7. no problem