Can I assume that $(D*D)/M≈ 0$ if $M>>D$?

Question

I am solving the problem attached, but I do not understand how the writer go from $(D(D+M))/M$ to $(D+1)$ in the last line of $1.16$. I am not sure if this is legal $(D*D)/M≈ 0$, because $D$ is quadratic and $M$ linear.

Answer 1

The writer went from $1+\dfrac{D(D+M)}M$ to $1+D$, because,

if $D\ll M$, then $\dfrac{D+M}M\approx1$, so $D\dfrac{D+M}M\approx D$.