Answer
a. $A(t)=P(1+\displaystyle \frac{r}{n})^{nt}$
b. $A(t)=Pe^{rt}$
Work Step by Step
Compound lnterest, p. 334:
If a principal $P$ is invested in an account paying an annual interest rate $r$,
compounded $n$ times a year,
then after $t$ years the amount $A(t)$ in the account is
$A(t)=P(1+\displaystyle \frac{r}{n})^{nt}$
Continuously compounded interest, p.340$:$
If the interest is compounded continuously, then the amount is
$A(t)=Pe^{rt}$
