Pr 20, Sec 2.7 Use Definition 4 to find f'(a) at the given number a. f ( x ) = 5 x4 a = -1. [ See Definition 4 above. ] Pr 26, Sec 2.7 Find f'(a). X f ( X ) = 1 - 4XDerivative = Instantaneous Rate of Change = Slope of the Tangent Line Definition 4. The derivative of a function fat a number a, denoted by f'(a), is f (a + h) - f (a) f' (a) = lim h -0 h