Chapter 3.7: Derivatives of the Trigonometric Functions
Chapter 3.7: Derivatives of the Trigonometric Functions
Expected Skills:
? Know (and be able to derive) the derivatives of the 6 elementary trigonometric functions.
? Be able to use the product, quotient, and chain rules (where appropriate) to differentiate functions involving trigonometry.
? Be able to use the derivative to calculate the instantaneous rates of change of a trigonometric function at a given point.
? Be able to use the derivative to calculate the slope of the tangent line to the graph of a trigonometric function at a given point.
? Be able to use the derivative to calculate to answer other application questions, such as local max/min or absolute max/min problems.
Practice Problems:
1. Fill in the given table:
f (x) sin x cos x tan x cot x sec x csc x
f (x)
d 2. Use the definition of the derivative to show that (cos x) = - sin x
dx Hint: cos ( + ) = cos cos - sin sin
3. Use the quotient rule to show that d (cot x) = - csc2 x. dx
d 4. Use the quotient rule to show that (csc x) = - csc x cot x.
dx
tan 5. Evaluate lim
3
+
h
- tan
3
by interpreting the limit as the derivative of a
h0
h
function at a particular point.
1
For problems 6-16, differentiate
6. f (x) = 2 cos x + 4 sin x
sin2 x 7. f (x) =
cos x 8. f (x) = x3 sin x
9. f (x) = sec2 x + tan2 x
1 10. f (x) = tan x2
11. f (x) = sec 2x
12. f (x) = cos3 3x
13. f (x) = sin x
14. f (x) = sin (sin 2x)
15. f (x) = tan2 (x2 - 1)
16. f (x) = 4x2 csc 5x
d
17. Use the following table to calculate g 2 sin x
dx
4
x=3
x f (x) f (x) g(x) g (x) 1 -2 -5 3 9 2 5 -3 4 -2 3 -1 6 7 -6 4 3 1 -2 5 54 7 1 8
18. What is the 100th derivative of y = sin (2x)?
cos x
19. Compute an equation of the line which is tangent to the graph of f (x) =
at the
x
point where x = .
20. Find all points on the graph of y = sin2 x where the tangent lines are parallel to the line y = x.
2
For problems 21-22, find all values of x in the interval [0, 2] where the graph of the given function has horizontal tangent lines.
21. f (x) = sin x cos x
22. g(x) = csc x
23. 24.
Use the Intermediate ValuewithTploths eorem to show that there is at least one point in the
animate, animate3d, animatecurve, arrow, changecoords, complexplot, complexplot3d,
(1)
1 3 conformal, conformal3d, contourplot, contourplot3d, coordplot, coordplot3d, densityplot, interval (0, 1) where the graph of f (x) = sin x - x will have a horizontal tangent display, dualaxisplot, fieldplot, fieldplot3d, gradplot, gradplot3d, implicitplot, implicitplot3d,
3 inequal, interactive, interactiveparams, intersectplot, listcontplot, listcontplot3d,
line.
listdensityplot, listplot, listplot3d, loglogplot, logplot, matrixplot, multiple, odeplot, pareto, plotcompare, pointplot, pointplot3d, polarplot, polygonplot, polygonplot3d,
polyhedra_supported, polyhedraplot, rootlocus, semilogplot, setcolors, setoptions,
setoptions3d, spacecurve, sparsematrixplot, surfdata, textplot, textplot3d, tubeplot
Consider the graphs of f (x) = 2 cos(x) and g(x) = a d plot sqrt 2 sin x , x = 0.. Pi , scaling = constrained 2 PLOT ...
interval 0, .
b d plot sqrt 2 cos x , x = 0 .. Pi , scaling = constrained
2
2 PLOT ...
2 sin(x) shown below on the
(2)
(3)
display a, b
1.4
1.2
1
0.8
0.6
0.4
0.2
0
3 5 3 7
16
8 16
4 16
8
16
2
x
Show that the graphs of f (x) and g(x) intersect at a right angle when x = . (Hint:
4
Show that the tangent lines to f and g at x = are perpendicular to each other.)
4
25. A 15 foot ladder leans against a vertical wall at an angle of with the horizontal, as shown in the figure below. The top of the ladder is h feet above the ground. If the ladder is pushed towards the wall, find the rate at which h changes with respect to at the instant when = 30. Express your answer in feet/degree.
3
26. Multiple Choice: At how many points on the interval [-, ] is the tangent line to the graph of y = 2x + sin x parallel to the secant line which passes through the graph endpoints of the interval? (a) 0 (b) 1 (c) 2 (d) 3 (e) None of these
For problems 27-28, use the Second Derivative Test to determine the relative (local) extrema. Express each as an ordered pair (x, y).
27. f (x) = sin (3x) on [0, ] 28. f (x) = sec (3x) on [0, ] For problems 29-30, Find all absolute extrema of the given function on the given interval. 29. f (x) = cos x - sin x on the interval [-, ].
30. f (x) = tan x + sin x on the interval - , .
44
4
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