an=am·q^(n-m)（m,n∈N*）∴an=a5·q^(n-5)=-8·(-1/2)^(n-5)=-128·(-1/2)^(n-1)
a1=a5÷q^4=-128an=a1×q^（n-1）=-128×（-1/2)^(n-1)
