Related Rates Seesaw Problem: Finding Average Rate of Change

[stokes]

1. Homework Statement
A child weighs 34 Kg is seated on a seesaw. While a child who weighs 40 kg is situated on the opposite end of the seesaw. The function B(x)= 34x / 40 gives the distance that the 40 kg child must sit from the center of the seesaw when the 34 kg child sits x meters from the center. The seesaw is 9m long find the average rate of change in distance as the lighter childs distance changes from 1.5m to 2.5meters.


2. Homework Equations
I used the quotient rule to find the derivative of the function. Which turns out to be 68 / 80.


3. The Attempt at a Solution
The thing is the derivative is 34/40. Which equals to 0.85.

The prof gave us the answer which is -1.275. I haven't come close to that.

I used...

f(x) = f(b) - f(a) / b-a


I don't know how to tackle the average rate of change.

 

[whiplash]

you have forgotten to use the given x value.

i.e.

34*x/40 = (34 * 1.5) / 40 = 1.275

 

[Shooting Star]

Hi stokes,

Was the problem formulated as you have given? Are you sure something's not missing?

 

[olgranpappy]

Hi stokes,

Was the problem formulated as you have given? Are you sure something's not missing?

...that's what *I* said... the first time he posted this problem in a different forum:

https://www.physicsforums.com/showthread.php?t=207541

 

[Shooting Star]

stokes, it would be best to take help from your professor.

And please let us know how he solved it.