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did you draw a picture?

i dont think you need to use related rates for this question

Yup gives me a right angle triangle |dw:1354488913281:dw|

the store being the point where 5 km/h and 13km/h meet

you drew it wrong

you cant use speed as distance values

|dw:1354489054440:dw|

|dw:1354489176738:dw|

@MarcLeclair Have you found x yet?

No still trying hehe. Taking some time :(

Total time T = run time + row time
( 2- x ) / 13 + √ ( x² + 9 ) / 5
Can you take derivative:

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

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

T' = x / 5√ ( x² + 9 ) - 1 / 13

Isn't mine right but not simplified?

T' = 0
x / 5√ ( x² + 9 ) = 1 / 13
5√ ( x² + 9 ) = 13x

Yes thank you chlorophyll :) I ll verify if my derivative was right. Thanks a lot for your help

It is pretty complicated, I've got more complicated to do haha. I'm enjoying it O_o

13x = 5√ ( x² + 9 )
-> 169x² = 25 ( x² + 9)
144x² = 225
x = 15/12

That's what I got too and it is right. You two were life saviour haha

=> x = 1hr 15'

Finally, plug x to find the distance, though :)
All credit should go to @completeidiot =)