Two vectors of lengths 5 and 8 have a dot product equal to –20. Which is true about the vectors?

The dot product is |u||v| cos θ. Since the vectors have lengths 5 and 8, their length product is 40, so cos θ = −20/40 = −1/2. A negative cosine means the angle is obtuse, so the true statement is “The angle between the vectors is obtuse.”

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