18: First, it is obvious that cosine does not cause any trouble and so it
can be evaluated separately. This still leaves a rather nasty product on the
top, which suggests that we break this into two fractions. However, it is
crucial to distribute the x3 wisely among the two
resulting fractions. If you put too many x into one and too few into
the other, the two resulting fractions could well yield the indeterminate
product
Now apply l'Hospital's rule.