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exponential maths question helppppp Watch

1. i completely forgot how to work these out =|
anyone help me out pleaseee

i need to work out k, ive forgotten how to use ln...

80 = 40e^21k

(e is raised to the power of 21k)
2. as we aren't allowed (meant) to give worked solutions on here... heres a hint...

natural logs...
3. i tried doing this:

ln2 / ln21 = k
but that didnt work out

then i tried doing

ln2 /ln21 = lnk ...but that doesnt make sense...

so not really sure where im going with this

i got the ln 2, from doing 80/40
4. (Original post by disisme)
i tried doing this:

ln2 / ln21 = k
but that didnt work out

then i tried doing

ln2 /ln21 = lnk ...but that doesnt make sense...

so not really sure where im going with this

i got the ln 2, from doing 80/40

Where did u get the ln21 from? Remember : as ln and e are inverse functions of each other.
5. (Original post by disisme)
i tried doing this:

ln2 / ln21 = k
but that didnt work out

then i tried doing

ln2 /ln21 = lnk ...but that doesnt make sense...

so not really sure where im going with this

i got the ln 2, from doing 80/40
Don't forget that the natural log cancels with the exponential so its actually 21k = ln2.
6. (Original post by didgeridoo12uk)
as we aren't allowed (meant) to give worked solutions on here... heres a hint...

natural logs...

okay so i found this written in my notebook, i did it last year..but cannot remember what i did/how to do it!!!

i wrote..

lne^21k = ln2
21lnk=ln2
21k=ln2

which gives me the right answer....but how/why did i get rid of the ln from the 21k???!!!
7. (Original post by JBKProductions)
Don't forget that the natural log cancels with the exponential so its actually 21k = ln2.

ooohhh okay thanks!! that was my next post lol..i didnt realise that!
cool tinnnngs
8. (Original post by disisme)
okay so i found this written in my notebook, i did it last year..but cannot remember what i did/how to do it!!!

i wrote..

lne^21k = ln2
21lnk=ln2
21k=ln2

which gives me the right answer....but how/why did i get rid of the ln from the 21k???!!!
that line is wrong!

basically you need to make use of the two rules...

or

depending on how you want to think about it

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