In fact ,you can say any nuber N,只要您能说出道理.比如

令 f(1) = 1/2; f(2) = 1/4; f(3) = 1/4; f(4) = N,确定唯一的三次函数:
f(x) = a*x^3 + b*x^2 + c*x + d.

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