[tex]\bf \begin{array}{llll}
term&value\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
a_1&40\\\\
a_2&0.5(a_{n-1})\\
&0.5(a_{2-1})\\
&0.5(a_1)\\
&0.5(40)\\
&20\\\\
a_3&0.5(a_{3-1})\\
&0.5(a_2)\\
&0.5(20)
\end{array}[/tex]
