What must be true to find the sum of a infinite sequence?

Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a certain commodity satisfy the demand equation 4p cubedplusx squaredequals38 comma 400. determine the rate at which sales are changing at a time when xequals80, pequals20, and the price is falling at the rate of $.20 per week.

Describe the 2 algebraic methods you can use to find the zeros of the function f(t)=-16t^2+400.

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