We have been given that
[tex]a_1=1\\ a_2=3\\
a_ n =(-1)^n a _{n-2} \text{ for } n>3[/tex]
In order to find the third term, we substitute n=3
[tex]a_ 3 =(-1)^3a _{(3-2)}\\
\\
a_ 3 =(-1)^3 a _{1}[/tex]
Substituting the given value of [tex]a_1[/tex]
[tex]a_ 3 =(-1)^3 \times 1\\
\\
a_3=-1\times 1\\
\\
a_3=-1[/tex]
Similarly, we can find the fourth term by plugging n=4
[tex]a_ 4 =(-1)^4a _{(4-2)}\\
\\
a_ 4 =(-1)^4 a _{2}\\
\\
a_4= 1\times 3\\
\\
a_4=3[/tex]
Therefore, the third term is -1
And the fourth term is 3
