# Compute the indicated derivative...just need guidance

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• Nov 13th 2012, 11:17 AM
JDS
Compute the indicated derivative...just need guidance
I worked out my solution and was hoping for some input from the experts as to whether I did this correctly or not.

Thanks in advance!!

Compute the indicated derivative.

f'''(t) for f(t) = 4t2 - 12 + (4/t2)

F’(t) = (d/dt) (4t2 -12 + (4/t2)

= 8t + 4t-2

F’’(t) = (d/dt) (8t + 4t-2)

= 8 – 8t-1

F’’’(t) = (d/dt) (8 – 8t-1)

= 8
• Nov 13th 2012, 11:42 AM
Plato
Re: Compute the indicated derivative...just need guidance
Quote:

Originally Posted by JDS
Compute the indicated derivative.
f'''(t) for f(t) = 4t2 - 12 + (4/t2)

It is completely wrong.
Write as: \$\displaystyle 4t^2-12+4t^{-2}\$
Then
\$\displaystyle 8t-8t^{-3}\$
\$\displaystyle 8+24t^{-4}\$
\$\displaystyle -96t^{-5}\$
• Nov 13th 2012, 11:47 AM
JDS
Re: Compute the indicated derivative...just need guidance
Wow, I don't know how I was so wrong! I think I need to go back and reread this chapter!!!
• Nov 13th 2012, 12:06 PM
JDS
Re: Compute the indicated derivative...just need guidance
Okay, so lets see if I understand this now...., lets use the following:

Find the derivative:

f(x) = (x^3/2 - 4x) (x^4 - 3/x^2 +2)

then I would rewrite as........

= (x^3/2 -4x) (x^4 - 3x^-2 + 2)

then, take the derivative.......

f ' (x) = [((3/2 x^-3/2) - 4)(x^4 - 3x^-2 +2)] + [(x^3/2 - 4x) (4x^2 + 3x^-3)]

Ive got a headache now, btw :(
• Nov 14th 2012, 06:51 AM
JDS
Re: Compute the indicated derivative...just need guidance
bump
• Nov 14th 2012, 08:37 AM
HallsofIvy
Re: Compute the indicated derivative...just need guidance
Quote:

Originally Posted by JDS
Okay, so lets see if I understand this now...., lets use the following:

Find the derivative:

f(x) = (x^3/2 - 4x) (x^4 - 3/x^2 +2)

then I would rewrite as........

= (x^3/2 -4x) (x^4 - 3x^-2 + 2)

then, take the derivative.......

f ' (x) = [((3/2 x^-3/2) - 4)(x^4 - 3x^-2 +2)] + [(x^3/2 - 4x) (4x^2 + 3x^-3)]

Not quite, though the errrors may be typos. The derivative of x^(3/2) is (3/2)x^(3/2- 1)= (3/2)x^(1/2), not (3/2)x^(-3/2),
the derivative of x^4 is 4x^(4- 1)= 4x^3, not 4x^2, and the derivative of 3x^(-2) is 3(-2)x^(-2-1)= -6x^(-3), not 3x^(-3).

Quote:

Ive got a headache now, btw :(
Don't worry, you'll get used to it!
• Nov 14th 2012, 09:03 AM
JDS
Re: Compute the indicated derivative...just need guidance
Thanks! I've been pulling my hair out over this, lol!