f = a·2^(b·t)
t = 0 : f = 250 batteri
t = 3 h : f = 500 batteri (raddoppiano)
250 = a·2^(b·0)---> a = 250 batteri iniziali
250·2 = 250·2^(b·3)---> b = 1/3
f = 250·2^(1/3·t)
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f = 4·250 = 1000 batteri
1000 = 250·2^(1/3·t)----> t = 6 h
y(t) = A b^t
y(0) = 250 = A*b^0 = A
y(t) = 250 * b^t
250*2 = 250*b^3
b^3 = 500/250 = 2
b = 2^(1/3)
y(t) = 250 * 2^(t/3)
250 * 4 = 250 * 2^(T/3)
2^(T/3) = 2^2
T/3 = 2
T = 6 h