1) The derivative of a constant is 0. The power rule states that when taking the derivative of a term with an x to a power, the power is subtracted by 1 and brought in front of the term. 2) When evaluating a derivative, you take the derivative of the function and plug in a value for x. For example, the derivative of f(x)=x^2 evaluated at x=3 is 2x=6. 3) In business and economics, the derivative is used to find marginal costs, revenues, and profits which provide the change in each per additional unit.