Answer:
Therefore rate of payment = $ 3145.72
Therefore the rate of interest = =$1573.17
Step-by-step explanation:
Consider A represent the balance at time t.
A(0)=$ 7864.
r=13 % =0.13
Rate payment = $k
The balance rate increases by interest (product of interest rate and current balance) and payment rate.
![\frac{dB}{dt} = rB-k](https://tex.z-dn.net/?f=%5Cfrac%7BdB%7D%7Bdt%7D%20%3D%20rB-k)
.......(1)
To solve the equation ,we have to find out the integrating factor.
Here p(t)= the coefficient of B =-r
The integrating factor ![=e^{\int p(t) dt](https://tex.z-dn.net/?f=%3De%5E%7B%5Cint%20p%28t%29%20dt)
![=e^{\int (-r)dt](https://tex.z-dn.net/?f=%3De%5E%7B%5Cint%20%28-r%29dt)
Multiplying the integrating factor the both sides of equation (1)
![e^{-rt}\frac{dB}{dt} -e^{-rt}rB=-ke^{-rt}](https://tex.z-dn.net/?f=e%5E%7B-rt%7D%5Cfrac%7BdB%7D%7Bdt%7D%20-e%5E%7B-rt%7DrB%3D-ke%5E%7B-rt%7D)
![\Rightarrow e^{-rt}dB - e^{-rt}rBdt=-ke^{-rt}dt](https://tex.z-dn.net/?f=%5CRightarrow%20%20e%5E%7B-rt%7DdB%20-%20e%5E%7B-rt%7DrBdt%3D-ke%5E%7B-rt%7Ddt)
Integrating both sides
![\Rightarrow \int e^{-rt}dB -\int e^{-rt}rBdt=\int-ke^{-rt}dt](https://tex.z-dn.net/?f=%5CRightarrow%20%5Cint%20e%5E%7B-rt%7DdB%20-%5Cint%20e%5E%7B-rt%7DrBdt%3D%5Cint-ke%5E%7B-rt%7Ddt)
[ where C arbitrary constant]
![\Rightarrow B(t)=\frac{k}{r} +Ce^{rt}](https://tex.z-dn.net/?f=%5CRightarrow%20B%28t%29%3D%5Cfrac%7Bk%7D%7Br%7D%20%2BCe%5E%7Brt%7D)
Initial condition B=7864 when t =0
![\therefore 7864= \frac{k}{r} - Ce^0](https://tex.z-dn.net/?f=%5Ctherefore%207864%3D%20%5Cfrac%7Bk%7D%7Br%7D%20-%20Ce%5E0)
![\Rightarrow C= \frac{k}{r} -7864](https://tex.z-dn.net/?f=%5CRightarrow%20%20C%3D%20%5Cfrac%7Bk%7D%7Br%7D%20-7864)
Then the general solution is
![B(t)=\frac{k}{r}-( \frac{k}{r}-7864)e^{rt}](https://tex.z-dn.net/?f=B%28t%29%3D%5Cfrac%7Bk%7D%7Br%7D-%28%20%5Cfrac%7Bk%7D%7Br%7D-7864%29e%5E%7Brt%7D)
To determine the payment rate, we have to put the value of B(3), r and t in the general solution.
Here B(3)=0, r=0.13 and t=3
![B(3)=0=\frac{k}{0.13}-( \frac{k}{0.13}-7864)e^{0.13\times 3}](https://tex.z-dn.net/?f=B%283%29%3D0%3D%5Cfrac%7Bk%7D%7B0.13%7D-%28%20%5Cfrac%7Bk%7D%7B0.13%7D-7864%29e%5E%7B0.13%5Ctimes%203%7D)
![\Rightarrow- 0.48\frac{k}{0.13} +11614.98=0](https://tex.z-dn.net/?f=%5CRightarrow-%200.48%5Cfrac%7Bk%7D%7B0.13%7D%20%2B11614.98%3D0)
⇒k≈3145.72
Therefore rate of payment = $ 3145.72
Therefore the rate of interest = ${(3145.72×3)-7864}
=$1573.17