



 
Hi Marina, I redrew your diagram so that I can label it.
From the diagram r + 2x = 100 km so r = 100  2x km. Also sin(θ) = 2/f so f = 2/sin(θ). Write x in terms of a trig function of θ. Using these relationships you can write your expression for the time, T = 2f/3 + r/8 as a function of θ. Now use your knowledge of calculus to find the value of θ that minimizes T. Write back if you need further assistance,  


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