When we talk about the function f defined for all real numbers x by

DERIVATIVES OF TRIGONOMETRIC FUNCTIONS When we talk about the function f defined for all real numbers x by , it is understood that means the sine of the angle whose radian measure is . A similar convention holds for the other trigonometric functions cos, tan, csc, sec, and cot

For first quadrant all, are positive so we can write Divide it by Take the inverse By the Squeeze Theorem, we have: However, the function (sin )/ is an even function. So, its right and left limits must be equal. Hence, we have:

Example:

Using the same methods as in the case of finding derivative of , we can prove:

The tangent function can also be differentiated by using the definition of a derivative. However, it is easier to use the Quotient Rule together with formulas for derivatives of as follows.

The derivatives of the remaining trigonometric functions — csc, sec, and cot — can also be found easily using the Quotient Rule. All together:

Example: For what values of x does the graph of f have a horizontal tangent? Differentiate

Since sec x is never 0, we see that f’(x) = 0 when tan x = 1. • This occurs when x = nπ +π/4, where n is an integer

Example: Find Example: Calculate:

CHAIN RULE How to differentiate composite function • The differentiation formulas you learned by now do not enable you to calculate F’(x). It turns out that the derivative of the composite function f ◦ g is the product of the derivatives of f and g. Proof goes over the head, so forget about that. This fact is one of the most important of the differentiation rules. It is called the Chain Rule.

It is convenient if we interpret derivatives as rates of change. • Regard: • as the rate of change of u with respect to x • as the rate of change of y with respect to u • as the rate of change of y with respect to x If u changes twice as fast as x and y changes three times as fast as u, it seems reasonable that y changes six times as fast as x. So,we expect that:

Chain Rule • If is differentiable at and is differentiable at , the composite function defined by is differentiable at andis given by the product: The Chain Rule can be written either in the prime notation or, if y = f(u) and u = g(x), in Leibniz notation: easy to remember because, if dy/du and du/dx were quotients, then we could cancel du. However, remember: du/dx should not be thought of as an actual quotient

How to differentiate composite function Let’s go back: In order not to make your life too complicated (it is already enough), we’ll introduce one way that is most common and anyway, everyone ends up with that one: Leibnitz Let where • dy/dx refers to the derivative of y when y is considered as a function of x • (called the derivative of y with respect to x) • dy/du refers to the derivative of y when considered as a function of u • (the derivative of y with respect to u)

example: • Differentiate: • )

example: Differentiate y = (x3 – 1)100 Taking u= x3 – 1 and y = u100

example: Find f’ (x) if • First, rewrite f as f(x) = (x2 + x + 1)-1/3 • Thus,

example: Find the derivative of • Combining the Power Rule, Chain Rule, and Quotient Rule, we get:

example: • Differentiate: y = (2x + 1)5 (x3 – x + 1)4 • In this example, we must use the Product Rule before using the Chain Rule.

The reason for the name ‘Chain Rule’ becomes clear when we make a longer chain by adding another link. Suppose that y = f(u), u = g(x), and x = h(t), where f, g, and h are differentiable functions, then, to compute the derivative of y with respect to t, we use the Chain Rule twice:

example: • Notice that we used the Chain Rule twice.

example: Differentiate

Divide both sides by The slope of a parametrized curve is given by: The chain rule enables us to find the slope of parametrically defined curves x = x(t) and y = y(t):

When we talk about the function f defined for all real numbers x by