



 
Hi Jenny, I drew a diagram and labeled it.
F is the position of the first car and S the position of the second car, both at some time t hours. f(t) is the distance of the first car from P at time t and s(t) is the distance od the second car from P at time t. Use the Law of Cosines to write x(t)^{2} in terms of f(t) and s(t). Differentiate this expression with respect to t to obtain an expression of the form
At the specific time t* when f(t*) = 40 km and s(t*) = 50 km you can evaluate the right side. Also at t* you can evaluate x(t*) using the Law of Cosines. This then allows you to evaluate x'(t*). Harley  


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