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MarcLeclair

  • 3 years ago

A sherman is in a rowboat on a lake and 3 km from shore. He wishes to reach a store 2 km down the (straight) shore. He can row at 5 km/h and run at 13 km/h. To what point down-shore should he row to get to the store as quickly as possible? It's supposed to be optimization but I'm wondering if I need to use related rates... :/

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  1. completeidiot
    • 3 years ago
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    did you draw a picture?

  2. completeidiot
    • 3 years ago
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    i dont think you need to use related rates for this question

  3. MarcLeclair
    • 3 years ago
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    Yup gives me a right angle triangle |dw:1354488913281:dw|

  4. MarcLeclair
    • 3 years ago
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    the store being the point where 5 km/h and 13km/h meet

  5. completeidiot
    • 3 years ago
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    you drew it wrong

  6. completeidiot
    • 3 years ago
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    you cant use speed as distance values

  7. MarcLeclair
    • 3 years ago
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    im just showing the distance where the speed is going. I thought it was normal to do. ( well for me to understand)

  8. completeidiot
    • 3 years ago
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    |dw:1354489054440:dw|

  9. completeidiot
    • 3 years ago
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    |dw:1354489176738:dw|

  10. MarcLeclair
    • 3 years ago
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    Oh yeah ended the triangle too late. I need to find the the point where he reaches the shore, But I usually get 2 equations, ( one is my constraint the other is the one I'm trying to maximize/minimize) I can't figure them out here Perimeter, or area, I mean I feel as if I don't have enough variables.

  11. completeidiot
    • 3 years ago
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    d/v=t |dw:1354489441544:dw| first determine the rowing distance \[d_{row}=\sqrt{x^2+9}\] and running distance \[d_{run}=2-x\] now calculate total time to travel said distance \[t= d_{row}/v_{row} + d_{run}/v_{run}\] you should get a nice fun equation then solve for x using first derivative then solve for t

  12. MarcLeclair
    • 3 years ago
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    Ahhhhhh so its not always with area and peremeter.... stupid me. THanks a lot , I'll try it from there on my own :P Gotta love math

  13. Chlorophyll
    • 3 years ago
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    @MarcLeclair Have you found x yet?

  14. MarcLeclair
    • 3 years ago
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    No still trying hehe. Taking some time :(

  15. Chlorophyll
    • 3 years ago
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    Total time T = run time + row time ( 2- x ) / 13 + √ ( x² + 9 ) / 5 Can you take derivative:

  16. MarcLeclair
    • 3 years ago
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    yeah thats what im trying to do right now, I got (-13x/13 ^2) +3(1/2( x ^2+9) ^(-1/2) (2x) / 9

  17. MarcLeclair
    • 3 years ago
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    er I dont mean 3 I mean 5 sorry got confuse in the numbers. so 5(1/2 * and the last one is /15

  18. Chlorophyll
    • 3 years ago
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    T' = x / 5√ ( x² + 9 ) - 1 / 13

  19. MarcLeclair
    • 3 years ago
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    Isn't mine right but not simplified?

  20. Chlorophyll
    • 3 years ago
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    T' = 0 x / 5√ ( x² + 9 ) = 1 / 13 5√ ( x² + 9 ) = 13x

  21. MarcLeclair
    • 3 years ago
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    Yes thank you chlorophyll :) I ll verify if my derivative was right. Thanks a lot for your help

  22. MarcLeclair
    • 3 years ago
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    It is pretty complicated, I've got more complicated to do haha. I'm enjoying it O_o

  23. Chlorophyll
    • 3 years ago
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    13x = 5√ ( x² + 9 ) -> 169x² = 25 ( x² + 9) 144x² = 225 x = 15/12

  24. MarcLeclair
    • 3 years ago
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    That's what I got too and it is right. You two were life saviour haha

  25. Chlorophyll
    • 3 years ago
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    => x = 1hr 15'

  26. MarcLeclair
    • 3 years ago
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    its okay hahahaha, everybody have their strong points, you still helped with calculations :) I'm aweful at making relationships in math :P

  27. Chlorophyll
    • 3 years ago
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    Finally, plug x to find the distance, though :) All credit should go to @completeidiot =)

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