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Calculus- Early Transcendentals, 2021a

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136 Derivatives<br />

since<br />

sinx<br />

cosx − 1<br />

lim = 1and lim = 0.<br />

x→0 x<br />

x→0 x<br />

♣<br />

A formula for the derivative of the cosine function can be found in a similar fashion:<br />

d<br />

dx (cosx)=−sinx.<br />

Using the quotient rule we get formulas for the remaining trigonometric ratios. To summarize, here<br />

are the derivatives of the six trigonometric functions:<br />

d<br />

dx (sin(x)) = cos(x) d<br />

dx (tan(x)) = sec2 (x)<br />

d<br />

(csc(x)) = −csc(x)cot(x)<br />

dx<br />

d<br />

dx (cos(x)) = −sin(x) d<br />

dx (cot(x)) = −csc2 (x)<br />

d<br />

(sec(x)) = sec(x)tan(x)<br />

dx<br />

Example 4.29: Derivative of Product of Trigonometric Functions<br />

Find the derivative of f (x)=sinxtanx.<br />

Solution. Using the Product Rule we obtain<br />

f ′ (x)=cosxtanx + sinxsec 2 x.<br />

♣<br />

Exercises for Section 4.4<br />

Exercise 4.4.1 Find the derivatives of the following functions.<br />

(a) sinxcosx (b) cotx (c) cscx − xtanx<br />

Exercise 4.4.2 Find the points on the curve y = x + 2cosx that have a horizontal tangent line.<br />

4.5 The Chain Rule<br />

Let h(x) = √ 625 − x 2 . The rules stated previously do not allow us to find h ′ (x). However, h(x) is a<br />

composition of two functions. Let f (x)= √ x and g(x)=625 − x 2 .Thenweseethat<br />

h(x)=(f ◦ g)(x).

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