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an=a1*q^n − 1
a2=a1*1/2^2-1
5/2=a1*1/2^1
5/2=a1*1/2 \:1/2
a1=5
a2=5/2
a3=5*1/2^3-1=5*1/2^2=5*1/4=5/4
a4=5*1/2^4-1=5*1/2^3=5*1/8=5/8
b)
a4=1/135 i q=-1/3
a4=a1*(-1/3)^4-1
1/135=a1*(-1/3)^3
1/135=a1*(-1/27) \:(-1/27)
a1=-1/5
a2=-1/5*(-1/3)^2-1=-1/5*(-1/3)^1=1/15
a3=-1/5*(-1/3)^3-1=-1/5*(-1/3)^2=-1/5*(1/9)=-1/45
a4=1/135