Quote Originally Posted by cm917 View Post
Sorry..coz I am not familiar with how to use the math symbol...so i didnt post it...

My work:

let e^t = u du/dt = e^t

My(t) = 1/18 u^2 / ( 1-3/2 u+5/9 u^2)
Applying quotient rule:
--> (1/9u-1/12 u ^2) / (1-2/3u+5/9u^2)^2

Should I let t=0...then....it turns out... (1/9-1/12 )/(1-3/2+5/9)^2 = 9
am I on the right track?

$\displaystyle \frac{dM}{dt} = \frac{dM}{du} \cdot \frac{du}{dt}$.

Your expression for $\displaystyle \frac{dM}{du}$ is correct and it's value at $\displaystyle t = 0 \Rightarrow u = 1$ is correct.

However, you clearly need to multiply that result by the value of $\displaystyle \frac{du}{dt}$ at $\displaystyle t = 0$. Since this multiplying factor is equal to 1, you did in fact get the correct final answer - but by luck ....