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**skeeter** let ship A begin at the origin. its position as a function of time is ...

$\displaystyle x = 0$

$\displaystyle y = -25t$

ship B starts at (70,0). its position as a function of time is ...

$\displaystyle x = 70$

$\displaystyle y = 15t$

using the distance formula, the distance between the two ships at any time t is ...

$\displaystyle D = \sqrt{(70-0)^2 + [15t - (-25t)]^2}$

$\displaystyle D = \sqrt{4900 + 1600t^2}$

$\displaystyle \frac{dD}{dt} = \frac{1600t}{\sqrt{4900+1600t^2}}$

now ... evaluate $\displaystyle \frac{dD}{dt}$ for $\displaystyle t = 6$ hrs