Answer:
a) - r=5%: ![S=$ 5,136.10](https://tex.z-dn.net/?f=S%3D%24%205%2C136.10)
- r=4%: ![S=$ 4,885.61](https://tex.z-dn.net/?f=S%3D%24%204%2C885.61%20%20)
- r=3%:
b) - r=5%: t=14 years
- r=4%: t=17 years [/tex]
- r=3%: t=23 years [/tex]
c) The amount obtained is
- Compuonded quarterly: $5,191.83
- Compuonded continously: $5,200.71
The latter is always greater, since the more often it is capitalized, the greater the effect of compound interest and the greater the capital that ends up accumulating.
Explanation:
The rate of accumulation of money is
![dS/dt=rS](https://tex.z-dn.net/?f=dS%2Fdt%3DrS)
To calculate the amount of money accumulted in a period, we have to rearrange and integrate:
![\int dS/S=\int rdt=r \int dt\\\\ln(S)=C*r*t\\\\S=C*e^{rt}](https://tex.z-dn.net/?f=%5Cint%20dS%2FS%3D%5Cint%20rdt%3Dr%20%5Cint%20dt%5C%5C%5C%5Cln%28S%29%3DC%2Ar%2At%5C%5C%5C%5CS%3DC%2Ae%5E%7Brt%7D)
When t=0, S=S₀ (the initial capital).
![S=S_0=Ce^{r*0}=Ce^0=C\\\\C=S_0](https://tex.z-dn.net/?f=S%3DS_0%3DCe%5E%7Br%2A0%7D%3DCe%5E0%3DC%5C%5C%5C%5CC%3DS_0)
Now we have the equation for the capital in function of time:
![S=S_0e^{rt}](https://tex.z-dn.net/?f=S%3DS_0e%5E%7Brt%7D)
a) For an initial capital of $4000 and for a period of five years, the amount of capital accumulated for this interest rates is:
- r=5%: ![S=4000e^{0.05*5}=4000*e^{0.25}= 5,136.10](https://tex.z-dn.net/?f=S%3D4000e%5E%7B0.05%2A5%7D%3D4000%2Ae%5E%7B0.25%7D%3D%205%2C136.10)
- r=4%: ![S=4000e^{0.04*5}=4000*e^{0.20}= 4,885.61](https://tex.z-dn.net/?f=S%3D4000e%5E%7B0.04%2A5%7D%3D4000%2Ae%5E%7B0.20%7D%3D%20%204%2C885.61%20%20)
- r=3%:
b) We can express this as
![S=S_0e^{rt}\\\\2S_0=S_0e^{rt}\\\\2=e^{rt}\\\\ln(2)=rt\\\\t=ln(2)/r](https://tex.z-dn.net/?f=S%3DS_0e%5E%7Brt%7D%5C%5C%5C%5C2S_0%3DS_0e%5E%7Brt%7D%5C%5C%5C%5C2%3De%5E%7Brt%7D%5C%5C%5C%5Cln%282%29%3Drt%5C%5C%5C%5Ct%3Dln%282%29%2Fr)
- r=5%: ![t=ln(2)/0.05=14](https://tex.z-dn.net/?f=t%3Dln%282%29%2F0.05%3D14)
- r=4%: ![t=ln(2)/0.04=17](https://tex.z-dn.net/?f=t%3Dln%282%29%2F0.04%3D17%20%20)
- r=3%:
c) When the interest is compuonded quarterly, the anual period is divided by 4. In 5 years, there are 4*5=20 periods of capitalization. The annual rate r=0.0525 to calculate the interest is also divided by 4:
![S = 4000 (1+(1/4)(0.0525))^{5*4}=4000(1.013125)^{20}\\\\S=4000*1.297958= 5,191.83](https://tex.z-dn.net/?f=S%20%3D%204000%20%281%2B%281%2F4%29%280.0525%29%29%5E%7B5%2A4%7D%3D4000%281.013125%29%5E%7B20%7D%5C%5C%5C%5CS%3D4000%2A1.297958%3D%205%2C191.83)
If compuonded continously, we have:
![S=S_0e^{rt}=4000*e^{0.0525*5}=4000*1.3= 5,200.71](https://tex.z-dn.net/?f=S%3DS_0e%5E%7Brt%7D%3D4000%2Ae%5E%7B0.0525%2A5%7D%3D4000%2A1.3%3D%205%2C200.71)
The amount obtained is
- Compuonded quarterly: $5,191.83
- Compuonded continously: $5,200.71
The latter is always greater, since the more often it is capitalized, the greater the effect of compound interest and the greater the capital that ends up accumulating.