Morning Star LL
You can use l'hopital to evaluate the limit of his equation. But From (X → 0) lim (1-bcosx) = 0 doesn't seem right. He wants limit to be 1, which means the numerator and denominator should be equal after using l'hopital. He needs to differentiate the integral (chainrule) and that should be equal to (1-bcosx). I think the author got that one wrong, because b=1 would mean you have to use l'hopital again or you get an undefined limit.