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7-40.

Two roads intersect at right angles. Elizabeth, who is driving east, leaves the intersection traveling at a speed of mph. Two hours later, Nairi leaves the same intersection at a speed of mph heading north. How fast is the distance between the cars changing five hours after Elizabeth leaves? Homework Help ✎

Let the eastbound car be at a distance from the intersection and the northbound car at distance y.
Write an equation for the distance between them.

Implicitly differentiate the distance equation with respect to time. This will create a rate equation.

Evaluate.

mph