Why does the formula for latus rectum of ellipse simplify as shown?
Half the length of the latus rectum of the ellipse can be found with:
y = ± (b/a) √(b^2 )
which the text book simplifies to
y = ± b^2/a
why is it not ?
y = ± b|b|/a
Technically, it is but both possible are "included" via the $\pm$. Though even stronger is that your $b>0$ so that $|b|=b$.