Answer
$-a$
Work Step by Step
With $0.5=\frac{1}{2}$,
$\ln0.5 = \ln \left(\dfrac{1}{2} \right)$
Recall:
$\ln\left(\dfrac{M}{N}\right) = \ln M-\ln N$
Usin g the rule above gives:
$\ln \left(\dfrac{1}{2} \right) = \ln1-\ln2$
With $\ln1 =0 \hspace{20pt} \text{and} \hspace{20pt} \ln2=a$, then
$\ln1-\ln2 = 0-a=-a$
Therefore,
$\ln0.5 = \boxed{-a}$
