rearranging exponential eq. help

• Oct 30th 2009, 07:32 PM
tommoturbo
rearranging exponential eq. help
Hi Guys can anyone oint me in the right direction of making the exponetial part the subject. ie i need to solve for T

(theta)=(Theta0) [1-e^-t/T}
20 = 190x[1-e^40/T]

im ok with transfroming eq's but not sure of the rules of this one

any help appreciated
• Oct 30th 2009, 07:47 PM
PatrickFoster
Is there a difference between "theta" and "Theta0"? I'm not sure what the first equation really is, it's confusing. I imagine that "Theta0" is the original theta value. I will solve for the second equation instead (by the way, is the time negative? -t = 40?)

$
20 = 190x(1 - e^{\frac{40}{T}})
$

$1 - \frac{20}{190x} = e^{\frac{40}{T}}$

$ln(1 - \frac{20}{190x}) = \frac{40}{T}$

$
T = \frac{40}{ln(1 - \frac{20}{190x})}
$

Patrick
• Oct 30th 2009, 08:40 PM
tommoturbo
Hi Patrick

thanks for taking the time to reply

That is spot on i did it another way working back over and got T = 360

but couldnt work out how to set it out like you have

Thankyou very much for your help