Now let's try subtracting a lower-order term:

We know cn - b $\in$ O(n).

Guess: T(n) = O(n)

Show: T(n) <= cn - b

Assume: T(n/2) <= cn/2 - b

T(n) = 2T(n/2) + 1

T(n) $\leq$ 2(cn/2 - b) + 1

= cn - 2b + 1 $\leq$ cn - b

True if b - 1 $\geq$ 0, b $\geq$ 1 (okay)