1 answer
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x^2 + y^2 = 25
2x dx/dt + 2y dy/dt = 0
2(-4) dx/dt + 2(3)(9) = 0
-8 dx/dt + 54 = 0
dx/dt = 54/8 = 27/4 = 6.75
makes sense; the particle is moving up to the right.