find 5 rational numbers between -1 and 0 by using the formula 1/2(A+B)

One number is 0 and other is -1 . Their middle term will be
 $\frac{1}{2}\left(0+\left(-1\right)\right) =\frac{1}{2}\times -1 =-0.5$

Now middle term of 0 and -0.5  will be
$\frac{1}{2}\left(0+\left(-0.5\right)\right) = \frac{1}{2}\left(-0.5\right) =-0.25$
Now middle term of  -1.0  and -0.5  will be 
​$\frac{1}{2}\left(-1.0+\left(-0.5\right)\right) = \frac{1}{2}\left(-1.5\right) =-0.75$
 middle term of  -1.0  and -0.75  will be 
​$\frac{1}{2}\left(-1.0+\left(-0.75\right)\right) = \frac{1}{2}\left(-1.75\right) =-0.875$
 middle term of  0  and -0.25  will be 
​$\frac{1}{2}\left(0+\left(-0.25\right)\right) = \frac{1}{2}\left(-0.25\right) =-0.125$
You can take any two number and find their middle value .In thus way you can find infinite number between them .

Hope this would have cleared your doubt .