a_4=-16
a_7=128
a_n=a_1*q^{n-1}
a_4=a_1*q^3
a_7=a_1*q^6
-16=a_1*q^3
a_1=\frac{-16}{q^3}
128=\frac{-16}{q^3}*q^6
128=-16*g^3
g^3=\frac{128}{-16}=-8
q=-2
a_1=\frac{-16}{-8}=2
a_9=a_1*q^8=2*(-2)^8=2*256=512
a_1=2
a_2=?
a_3=?
a_4=?
a_5=\frac{1}{8}
a_n=a_1*q^{n-1}
a_5=a_1*q^4
\frac{1}{8}=2*q^4
16q^4=1
q^4=\frac{1}{16}
q=\frac{1}{2}
a_2=2*\frac{1}{2}=1
a_3=2*(\frac{1}{2})^2=2*\frac{1}{4}=\frac{1}{2}
a_4=2*(\frac{1}{2})^3=2*\frac{1}{8}=\frac{1}{4}