Compound Interest
1. **State the problem:**
Bob invests 1100 dollars in a 3-year CD with an annual interest rate of 1.28%, compounded daily. We need to find:
(a) The amount of money in Bob's account after 3 years.
(b) The interest earned after 3 years.
2. **Formula for compound interest with daily compounding:**
$$A = P \left(1 + \frac{r}{n}\right)^{nt}$$
where
- $A$ is the amount after time $t$,
- $P = 1100$ is the principal,
- $r = 0.0128$ is the annual interest rate as a decimal,
- $n = 365$ is the number of compounding periods per year (daily),
- $t = 3$ years.
3. **Calculate the amount after 3 years:**
$$A = 1100 \left(1 + \frac{0.0128}{365}\right)^{365 \times 3} = 1100 \left(1 + 0.000035068\right)^{1095}$$
Calculate the base inside the parenthesis:
$$1 + 0.000035068 = 1.000035068$$
Now raise to the power of 1095:
$$1.000035068^{1095} \approx e^{1095 \times \ln(1.000035068)}$$
Calculate the exponent:
$$1095 \times \ln(1.000035068) \approx 1095 \times 0.000035068 = 0.03838$$
Thus:
$$A \approx 1100 \times e^{0.03838} \approx 1100 \times 1.03915 = 1143.06$$
4. **Calculate the interest earned:**
$$\text{Interest} = A - P = 1143.06 - 1100 = 43.06$$
**Final answers:**
(a) Amount after 3 years: $1143.06$
(b) Interest earned after 3 years: $43.06$