Two fair 4-sided dice are rolled and the sum is noted (I) Find the probability of a sum of 7 given that the sum is at least 7 .

Consider the function f(x) = x ³. Since 2³ = 8 is a perfect cube that is pretty close to x = 1.999, take the linear approximation to f(x) at x = 2. It is

f(x) ≈ L(x) = f (2) + f ' (2) (x - 2)

Compute the derivative:

f(x) = x ³   →   f ' (x) = 3x ²   →   f ' (2) = 12

Then

L(x) = 8 + 12 (x - 2) = 12x - 16

Now approximate 1.999³ :

f (1.999) ≈ L (1.999) = 12•1.999 - 16 = 7.988

(Compare this to the actual value, 7.98801.)