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September 18, 2014

September 18, 2014

Posted by **Katie** on Sunday, February 6, 2011 at 4:59pm.

I think the answer is 55 mph because I added 60 + 55 and divided by 2 and got 55 mph.

- Calculus -
**Damon**, Sunday, February 6, 2011 at 5:09pmOh my :(

The distance between them is the hypotenuse of a right triangle.

D^2 = x^2 + y^2

where

x = 50 t

y = 60 t

so

D^2 = 2500 t^2 + 3600 t^2 = 6100 t^2

now take time derivative

2 D dD/dt = 12200 t

at t = 1.5

D^2 = 6100 (2.25)

D = 117 miles

2 D = 234.3 miles

so

dD/dt = 12200(1.5)/234.3 = 78.1 miles/hour

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