1. ## Another derivative problem

I'm just getting into derviatives as a rate of change.

The equation of a moving body is given by 3t^2 - 5t + 7 feet.

My book says that the velocity is the derivative of this equation and that is 6t-5

However I got something completely different when I derived the first equation.

Then the derivative of 6t-5 at t=3 ends up being 6. But I still don't get that.

What am I doing wrong?

2. ## Re: Another derivative problem

Originally Posted by Nervous
I'm just getting into derviatives as a rate of change.

The equation of a moving body is given by 3t^2 - 5t + 7 feet.

My book says that the velocity is the derivative of this equation and that is 6t-5

However I got something completely different when I derived the first equation.

Then the derivative of 6t-5 at t=3 ends up being 6. But I still don't get that.

What am I doing wrong?
position ...

$\displaystyle x(t) = 3t^2 - 5t + 7$

velocity is the derivative of position ...

$\displaystyle x'(t) = v(t) = 6t - 5$

$\displaystyle v(3) = 6(3) - 5 = 13$

3. ## Re: Another derivative problem

Originally Posted by Nervous
I'm just getting into derviatives as a rate of change.

The equation of a moving body is given by 3t^2 - 5t + 7 feet.

My book says that the velocity is the derivative of this equation and that is 6t-5

However I got something completely different when I derived the first equation.

Then the derivative of 6t-5 at t=3 ends up being 6. But I still don't get that.

What am I doing wrong?
Do you understand that we can't tell you what you did wrong when you haven't told us what you did?
What did you do and what result did you get?

(Do you know the standard formula for the derivative of x^n?)

4. ## Re: Another derivative problem

I figured it out...
I knew that both (3)' and (5)' equaled 0, but I didn't multiply zero by their neighbors when I applied the product rule.

5. ## Re: Another derivative problem

Originally Posted by Nervous
I figured it out...
I knew that both (3)' and (5)' equaled 0, but I didn't multiply zero by their neighbors when I applied the product rule.
huh ???