Geometric sequence
an = a1 r ^(n-1)
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f(n)=1/2*4^n, n≥0
in this case a1= 1/2
r = 4
n= 0 (you just replace n)
f( 0 )=1/2*4^0
f(0) = 1/2 =1/ 2
n f(n)
0 f(0) = 1/ 2 f( 0 )=1/2*4^0
1 f(1) = 2 f( 0 )=1/2*4^2 = 4/2
2 f(2) = 8 f( 0 )=1/2*4^2 = 16/2
3 f(3) = 32 f( 0 )=1/2*4^2 =64/2
4 f(4) = 128 f( 0 )=1/2*4^2 = 256/2
