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# Basic differentiation watch

1. The radioactive decay of a substance is given by:
R=1000e^-ct
t>0
, where R is the number of atoms at time t years and c is a positive constant.
Find the number of atoms when the substance started to decay?
2. put t=0. 1000.
3. (Original post by IsaacMed17)
put t=0. 1000.
why did you pick zero instead of other numbers? I don't get the logic behind it???
4. (Original post by Chelsea12345)
why did you pick zero instead of other numbers? I don't get the logic behind it???
I assume because the question asks "Find the number of atoms when the substance STARTED to decay".

The substance would start to decay at T = 0 years.
5. (Original post by Chelsea12345)
why did you pick zero instead of other numbers? I don't get the logic behind it???
Just in case you do not understand the other persons reason I am posting this.

T is for the time in that sense. T=0 makes a lot of sense when you are trying to find the initial rate. This is because it is at the start when the time has not passed yet and it is in its original radioactivity level.

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Updated: October 16, 2017
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