Respuesta :
Answer:
The speed of the boat in calm water is 12.8 miles per hour.
Step-by-step explanation:
While going downstream the speed of the boat is "x + 2", where x is the speed in calm water, while going upstream the speed of the boat is "x - 2". He made a trip that had two legs, each with a distance of 50 miles, therefore, the sum of the times it took him to complete each leg must be equal to the total time of the trip.
[tex]speed = \frac{distance}{time}\\\\time = \frac{distance}{speed}[/tex]
For the upstream:
[tex]time_{up} = \frac{50}{x - 2}[/tex]
For the downstream:
[tex]time_{d} = \frac{50}{x + 2}[/tex]
The sum of each of these times must be equal to "8 h", therefore:
[tex]8 = time_{up} + time_{d}\\\\8 = \frac{50}{x - 2} + \frac{50}{x + 2}\\\\8 = \frac{50*(x+2) + 50*(x - 2)}{(x-2)*(x+2)}\\\\8*(x-2)*(x+2) = 50*x + 100 + 50*x - 100\\\\8*(x^2 - 4) = 100*x\\\\8*x^2 - 100*x - 32 = 0\\\\x^2 - 12.5*x - 4 = 0\\\\x_{1,2} = \frac{-(-12.5) \pm \sqrt{(-12.5)^2 - 4*(1)*(-4)}}{2}\\\\x_{1,2} = \frac{12.5 \pm \sqrt{156.25 + 16}}{2}\\\\x_{1,2} = \frac{12.5 \pm \sqrt{172.25}}{2}\\\\x_{1,2} = \frac{12.5 \pm 13.124}{2}\\\\x_1 = 12.812\\\\x_2 = -0.312[/tex]
Since the speed can't be negative in this context, the only possible answer is 12.812 mph.
