[tex]t_1=-60[/tex]
[tex]t_2=-\dfrac12\times(-60)=30[/tex]
[tex]t_3=-\dfrac12\times30=\left(-\dfrac12\right)^2\times(-60)=-15[/tex]
In general, you have
[tex]t_n=\left(-\dfrac12\right)^{n-1}\times(-60)[/tex]
and so
[tex]t_{20}=\left(-\dfrac12\right)^{19}\times(-60)=\dfrac{15}{131072}\approx0.0001144[/tex]
