
In the proof for the inductive step, we start by assuming $k \ge 10$. But along the way, the author mentions $k \ge 1$ and $k \ge 7$ to justify the inequality.
Why do we bother to do this instead of just sticking with $k \ge 10$?

In the proof for the inductive step, we start by assuming $k \ge 10$. But along the way, the author mentions $k \ge 1$ and $k \ge 7$ to justify the inequality.
Why do we bother to do this instead of just sticking with $k \ge 10$?
Because in those cases all that was needed was to show that $k\geq1$ or $k\geq7$, which is obviously true if $k\geq10$. The author was simply stating the minimum required for the inequality to be true at each stage.