Answer
$-0.000000002$
Work Step by Step
For solving this, we will use the formula for the $nth$ term of the geometric series. $a_{n}=a_{1}\times r^{n-1}$.
Then put correct value for $n$ to solve.
$a_{40}=1000 \times (-\frac{1}{2})^{40-1}=-0.000000002$