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ktpr2

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Two cars A and B are connected by a rope 39 ft long that passes over a pulley P. The point Q is on the floor 12 ft directly beneath P and between the carts. Cart A is being pulled away from Q at a speed of 2 ft/s. How fast is cart B moving towards Q at the instant when cart A is 5 ft from Q?

The diagram they give is similar to an isocelles triangle with PQ running 12 ft down the middle. Point A is on the left, point B is on the right, representing carts A and B respectively.

I figure they want me to find [tex]\frac{dB}{dt} = \frac{dB}{dA} \frac{dA}{dt}[/tex] So i expressed point QB in terms of QA and got [tex]\sqrt{ (39-\sqrt{QA^2+144})^2-144}[/tex]. However this taking the deriviative of this times 2ft/s does not yeild the correct answer. What would be the correct way to relate QB in terms of QA?