Am i retarded or is something wrong?

[ManBearPig]

The derivative function of -5x^3 + 3x^2 + 8

=-15x^2+6x

right?

it wants me to find the equation of the tangent line at -2, but the only answer i can get is: 48x+132

how the hell is this wrong!?!!?

 

[bladder23]

Oh we're doing this in math too.

 

[BigJ]

I didn't try solving the problem, but remember what the derivative is.

You have your x value (-2), so find your y value. Now you have 3 out of 4 pieces you need to find the equation of the line.

 

[ManBearPig]

so you find x and y, then you find the derivative function, and then you put x into the derivative function to get m, and then you put that into y-y1=m(x-x1), right? i still cant get it.

 

[Itchrelief]

your slope should be -72, shouldn't it?

you use the derivative to find slope
y'(-2)=m=-72

you use the original equation to find the y-intercept
y(-2)=60
y=mx+b
60=-72(-2)+b
b=-84
y=-72x-84

i'm kinda drunk so might want to check that. i made a gross reading comprehension error earlier today

edit: this assumes that when you said "at 2" that the 2 is the x-value

 

[oiprocs]

Fool do your own HW. How old are you to come on ATOT with some simple work that could EASILY be found by doing a little google research.

:roll:

 

[darkswordsman17]

You know you're a nerd when you're "kinda drunk" and still do Calculus.

 

[jagec]

Originally posted by: Itchrelief
your slope should be -72, shouldn't it?

you use the derivative to find slope
y'(-2)=m=-72

you use the original equation to find the y-intercept
y(-2)=60
y=mx+b
60=-72(-2)+b
b=-84
y=-72x-84

i'm kinda drunk so might want to check that. i made a gross reading comprehension error earlier today

edit: this assumes that when you said "at 2" that the 2 is the x-value

:thumbsup:

From a sober person.

 

[ManBearPig]

err, that was absolutely right. how did i mess that up though?

i thought you did y-y1=m(x-x1)

which would be

y-60=-72(x- -2)
y=-72x-96

wtf

 

[BigJ]

Originally posted by: Kazaam
err, that was absolutely right. how did i mess that up though?

i thought you did y-y1=m(x-x1)

which would be

y-60=-72(x- -2)
y=-72x-96

wtf

-72x - 144. You add 60 and get -84.

y = -72x-84

 

[Brainonska511]

Originally posted by: Kazaam
err, that was absolutely right. how did i mess that up though?

i thought you did y-y1=m(x-x1)

which would be

y-60=-72(x- -2)
y=-72x-96

wtf

Um.... you're math is still wrong.

It should read:

y - 60 = -72(x + 2)
y = -72x - 144 + 60
y = -72x - 84

 

[ManBearPig]

ohhhhhhhhhhhhhhhhh. im a moron, thanks a lot. so i can do it either with y-y1=m(x-x1) or y=mx+b form?

thanks for the help guys (and girls)

 

[TuxDave]

Originally posted by: Kazaam
The derivative function of -5x^3 + 3x^2 + 8

=-15x^2+6x

right?

it wants me to find the equation of the tangent line at -2, but the only answer i can get is: 48x+132

how the hell is this wrong!?!!?

You messed up on the slope which probably fubared the rest.

Correct slope is:
-15*(-2)^2 + 6*(-2)=-60-12=-72

What you probably did was
-15*-(2)^2 + 6*-(2)=60-12 = +48

Negatives go inside the exponent!!!

 

[schneiderguy]

Originally posted by: bladder23
Oh we're doing this in math too.

were doing this also

Maybe we're all in the same class

 

[Lithium381]

Originally posted by: Brainonska511

Originally posted by: Kazaam
err, that was absolutely right. how did i mess that up though?

i thought you did y-y1=m(x-x1)

which would be

y-60=-72(x- -2)
y=-72x-96

wtf

Um.... you're math is still wrong.

It should read:

y - 60 = -72(x + 2)
y = -72x - 144 + 60
y = -72x - 84

Um.... your English is still wrong.

 

[ManBearPig]

so i can do it either with y-y1=m(x-x1) or y=mx+b form, right?

 

[Cogman]

Meh, We are just finding the center of gravity in my math class , I guess Im not in the cool kids calculus class (im in calc 2)

 

[Hyperlite]

Originally posted by: Cogman
Meh, We are just finding the center of gravity in my math class , I guess Im not in the cool kids calculus class (im in calc 2)

I am in Calc 2 as well...and i will be fighting tooth and nail to pass it.

 

[imported_quasi]

Originally posted by: Kazaam
so i can do it either with y-y1=m(x-x1) or y=mx+b form, right?

Yes. Y-intercept form (y = mx + b) is just point-slope form [y - y1 = m(x - x1)] solved for y.