Chapter 2.5 Practice Problems
Chapter 2.5 Practice Problems
EXPECTED SKILLS: ? Know the derivatives of the 6 elementary trigonometric functions. ? Be able to use these derivatives in the context of word 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)
f (x) sin x cos x tan x cot x sec x csc x
f (x) cos x - sin x sec2 x - csc2 x sec x tan x - csc x cot x
d 2. Use the definition of the derivative to show that (cos x) = - sin x
dx Hint: cos ( + ) = cos cos - sin sin
d
cos (x + h) - cos x
(cos x) = lim
dx
h0
h
cos x cos h - sin x sin h - cos x
= lim
h0
h
cos x cos h - cos x sin x sin h
= lim
-
h0
h
h
cos h - 1
sin h
= lim cos x
- sin x
h0
h
h
= (cos x)(0) - (sin x)(1)
= - sin x
1
3. Use the quotient rule to show that d (cot x) = - csc2 x. dx
d
d cos x
(cot x) =
dx
dx sin x
(sin x)(- sin x) - (cos x)(cos x)
=
sin2 x
-(sin2 x + cos2 x)
=
sin2 x
1 = - sin2 x
= - csc2 x
d 4. Use the quotient rule to show that (csc x) = - csc x cot x.
dx
d
d1
(csc x) =
dx
dx sin x
(sin x)(0) - (1)(cos x)
=
sin2 x
cos x
= - sin2 x
1 cos x =-
sin x sin x
= - csc x cot x
tan 5. Evaluate lim
3
+
h
- tan
3
by interpreting the limit as the derivative of a
h0
h
function at a particular point.
tan lim
3
+
h
- tan
3
d = (tan x)
= sec2 = 4
h0
h
dx
x=
3
3
For problems 6-14, differentiate
6. f (x) = 2 cos x + 4 sin x -2 sin x + 4 cos x
7. f (x) = 5 cos x + cot x -5 sin x - csc2 x
2
8. g(x) = 4 csc x + 2 sec x -4 csc (x) cot (x) + 2 sec (x) tan (x)
9. f (x) = sin x cos x cos2 x - sin2 x sin2 x
10. f (x) = cos x
2 sin x + sin x tan2 x 11. f (x) = x3 sin x
3x2 sin x + x3 cos x 12. f (x) = sec2 x + tan2 x
4 sec2 (x) tan (x) x + sec x
13. f (x) = 1 + cos x
1 + 2 tan x + cos x + sec (x) tan (x) + x sin x (1 + cos x)2 d2y
For problems 14-17, compute dx2 14. f (x) = tan x 2 sec2 x tan x 15. f (x) = sin x - sin x 16. f (x) = cos2 x 2 sin2 x - 2 cos2 x 17. f (x) = sin2 x + cos2 x 0
3
For problems 18-19, find all values of x in the interval [0, 2] where the graph of the given function has horizontal tangent lines.
18. f (x) = sin x cos x 3 5 7 , , , ; 44 4 4
19. g(x) = csc x
3 ,
22
20. 21.
cos x
Compute an equation of the line which is tangent to the graph of f (x) =
at the
x
point where x = .
with plots
12 y = 2 x -
animate, animate3d, animatecurve, arrow, changecoords, complexplot, complexplot3d,
(1)
conformal, conformal3d, contourplot, contourplot3d, coordplot, coordplot3d, densityplot,
display, dualaxisplot, fieldplot, fieldplot3d, gradplot, gradplot3d, implicitplot, implicitplot3d,
inequal, interactive, interactiveparams, intersectplot, listcontplot, listcontplot3d,
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) = 2 sin(x) shown below on the a dplot
sqrt
2
sin
x
,
x
=
0 ..
Pi 2
,
scaling
=
constrained
PLOT ...
(2)
interval 0, . b d plot sqrt 2 cos x , x = 0 .. Pi , scaling = constrained 22
PLOT ...
(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
f
= -1 and g
= 1. So, the tangent lines to f and g at x = are
4
4
4
perpendicular to one another since the product of their slopes is -1.
4
22. 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.
dh 15 3
3
=
ft/radian =
ft/degree
d 2
24
23. Use the Intermediate Value Theorem to show that there is at least one point in the interval (0, 1) where the graph of f (x) = sin x - 1 x3 will have a horizontal tangent 3 line.
f (x) = cos x - x2. Firstly, notice that f (x) is continuous for all x; therefore, it is continuous for all x in [0, 1]. Secondly, notice that f (0) = 1 > 0 and f (1) = cos (1) - 1 < 0. Thus, the Intermediate Value Theorem states there is at least one x0 in the interval (0, 1) with f (x0) = 0. In other words, there is at least one x0 in (0, 1) where f (x) will have a horizontal tangent line.
24. 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
C
5
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