Answer
$\dfrac{1}{2},\dfrac{-1}{4},\dfrac{1}{8},\dfrac{-1}{16}$
Work Step by Step
Need to find the first four terms of $a_n=\dfrac{(-1)^{n+1}}{2^n}$ when $n=1,2,3,4$ Thus, $a_1=\dfrac{(-1)^{1+1}}{2^1}=\dfrac{1}{2}$ ;
$a_2=\dfrac{(-1)^{2+1}}{2^2}=\dfrac{-1}{4}$ ;
$a_3=\dfrac{(-1)^{3+1}}{2^3}=\dfrac{1}{8}$ ;
$a_4=\dfrac{(-1)^{4+1}}{2^4}=\dfrac{-1}{16}$ ;
Hence, the first four terms are: $\dfrac{1}{2},\dfrac{-1}{4},\dfrac{1}{8},\dfrac{-1}{16}$
