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The mass, m grams, of a radioactive substance, present at time t days after first being observed, is given by the formula m=24e^-0.02t. Find
(i) the value of m when t=30.
(ii) the value of t when the mass is half of its value at t=0.
(iii) the rate at which the mass is decreasing when t=50.


Sagot :

Answer:

(i) The value of m when t = 30 is 13.2

(ii) The value of t when the mass is half of its value at t=0 is 34.7

(iii) The rate of the mass when t=50 is -0.18            

Step-by-step explanation:

(i) The m value when t = 30 is:

[tex] m = 24e^{-0.02t} = 24e^{-0.02*30} = 13.2 [/tex]

Then, the value of m when t = 30 is 13.2

(ii) The value of the mass when t=0 is:

[tex] m_{0} = 24e^{-0.02t} = 24e^{-0.02*0} = 24 [/tex]    

Now, the value of t is:

[tex] ln(\frac{m_{0}/2}{24}) = -0.02t [/tex]

[tex] t = -\frac{ln(\frac{24}{2*24})}{0.02} = 34.7 [/tex]

Hence, the value of t when the mass is half of its value at t=0 is 34.7

(iii) Finally, the rate at which the mass is decreasing when t=50 is:

[tex] \frac{dm}{dt} = \frac{d}{dt}(24e^{-0.02t}) = 24(e^{-0.02t})*(-0.02) = -0.48*                            (e^{-0.02*50}) = -0.18 [/tex]

Therefore, the rate of the mass when t=50 is -0.18.

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