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 ∞⋅0. Here the key observation comes from our rich experience with limits. We know that sin(x)/x goes to 1 at 0, so such a fraction does not cause any trouble. In other words, the best arrangement for the limit is

Now apply l'Hospital's rule.

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