This problem uses the chain rule.
y = f(x), then dy/dt = f'(x) dx/dt.y = 2x^3 - 4x
dy/dt = (2(3)x^2 - 4) dx/dt = (6x^2 - 4) * dx/dt
Since dx/dt = 4 and x = 1,
dy/dt = (6(1)^2 - 4)(4) = 8.
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