[RESOLVED]solving ln equation

[hani1987]

hi

my equation is r(t) = r0e^(-kt/3)

you need to solve for t, when r =0

the equation becomes lnr=-kt/3 +lnr0

but ln of 0 is undefined. Any ideas on how to solve would be appreciated.

[TBeck]

the only two times this equation will equal zero is if r0 is zero. this is because e^x will never equal zero for a finite value of x (in your case x = -kt/3)

the other way e^x = 0 is if you take the limit as x aproaches negative infinity, and in your case since x = -kt/3 then as x approaches negative infinity then t aproaches positive infinity, so r(t) = 0 when t approaches infinity

if you graph it then you get something like the attatched image.
(i graphed y=100*e^(-x/3) so r0 = 100 and k = 1)

[hani1987]

thanks

[TBeck]

no problem, glad to help.
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