3.5.1

Derivatives of Functions

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Derivatives

The derivative of a function, f(x), is the function that describes how the gradient changes as x changes. This is the same as saying that the derivative the rate of change of an equation.

Derivatives

Derivatives

  • The derivative of a function can be used to calculate the gradient of the function at any point.
  • The derivative of f(xx) is called f'(xx).
    • f'(xx) can be called "f dash of xx" or "f prime".
Example

Example

  • A function, f(xx), has the derivative, f'(xx) = 2xx + 5
  • What is the gradient of f(xx) when xx = 3?
    • Gradient at xx = 3 is f'(3).
    • f'(3) = (2 × 3) + 5 = 11
  • f(xx) has a gradient of 11 when xx = 3.

Differentiation

The derivative of a function can be worked out through a process called differentiation.

Differentiation

Differentiation

  • There are different ways to display the derivative of a function.
    • If your equation starts with y =, then the derivative of the equation is dydx\large\frac{dy}{dx}.
    • If your function starts with f(xx) =, then the derivative of the equation is f'(xx).
    • The exam may also ask you to find the rate of change, the gradient of the tangent or to differentiate a function.
General rule

General rule

  • To differentiate a whole equation/function, each term should be differentiated individually.
  • The general rule for differentiating a term is:
    • f(xx) = axnax^n → f'(xx) = naxnax nn−1
Examples

Examples

  • f(xx) = axnax^n → f'(xx) = naxnax nn−1
    • f(xx) = xx3 → f'(xx) = 3xx2
    • f(xx) = 2xx3 → f'(xx) = 6xx2
    • f(xx) = 6xx → f'(xx) = 6 (remember f(xx) = 6xx is the same as 6xx1)
    • f(xx) = 3xx4 → f'(xx) = 12xx3
    • f(xx) = 9 → f'(xx) = 0 (remember f(xx) = 9 is the same as 9xx0)
Example 1 with multiple terms

Example 1 with multiple terms

  • For each term in an expression, use f(xx) = axnax^n → f'(xx) = naxnax nn−1
    • f(xx) = xx3 + 2xx2 + 7xx
    • f'(xx) = 3xx2 + 4xx + 7
Example 1 with multiple terms

Example 1 with multiple terms

  • For each term in an expression, use f(xx) = axnax^n → f'(xx) = naxnax nn−1
    • f(xx) = 3xx4 + 5xx3 + 4xx2
    • f'(xx) = 12xx3 + 15xx2 + 8xx

Derivatives in Kinematics

Another use for the derivative is to analyse motion along a line.

Velocity

Velocity

  • The velocity of an object in motion is the rate of change of its displacement.
  • So the derivative of an object's displacement as a function of time is equal to the velocity, or:
    • v(t) = s'(t)
  • Where v(t) is the velocity and s(t) is the position, both as a function of time
Acceleration

Acceleration

  • Taking the derivative of the velocity gives the acceleration of the object, which is the rate of change of velocity.
    • a(t) = v'(t) = s''(t)
  • So the acceleration can also be thought of as the second derivative of the object's displacement with respect to time.
Jump to other topics
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Numbers

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Equations, Formulae & Identities

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Sequences, Functions & Graphs

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Geometry

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Vectors & Transformation Geometry

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Statistics & Probability

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