Chapter 5

Chapter 5

Exercise 5A

1. d x5 - x2 = 5x4 - 2x dx

2. d 3 + x3 = 0 + 3x2 dx = 3x2

d 3. (5 - cos x) = 0 - - sin x

dx = sin x

d 4. (sin x - cos x) = cos x - - sin x

dx = cos x + sin x

d 5. (cos x - sin x) = - sin x - cos x

dx

6. d (x - tan x) = 1 - (1 + tan2 x) dx = - tan2 x

d 7. ((x + 1)(2x - 3)) = 1(2x - 3) + 2(x + 1)

dx = 2x - 3 + 2x + 2

= 4x - 1

8. d 5x2(1 - 5x) = 10x(1 - 5x) - 5(5x2) dx = 10x - 50x2 - 25x2

= 10x - 75x2

d 9. (6 sin x) = (0)(sin x) + (6)(cos x)

dx = 6 cos x

d 10. (4 cos x) = (0)(cos x) + (4)(- sin x)

dx = -4 sin x

d 11. (x sin x) = (1)(sin x) + (x)(cos x)

dx = sin x + x cos x

12. d x2 cos x = (2x)(cos x) + (x2)(- sin x) dx = 2x cos x - x2 sin x

d 13.

dx

x 3x2 - 1

(1)(3x2 - 1) - (x)(6x)

=

(3x2 - 1)2

3x2 - 1 - 6x2 = (3x2 - 1)2

-3x2 - 1 = (3x2 - 1)2

3x2 + 1 = - (3x2 - 1)2

d 14.

dx

x2 + 1 x2 - 1

(2x)(x2 - 1) - (x2 + 1)(2x)

=

(x2 - 1)2

(2x)(x2 - 1 - x2 - 1)

=

(x2 - 1)2

(2x)(-2) = (x2 - 1)2

4x = - (x2 - 1)2

d 15.

dx

cos x x

(- sin x)(x) - (cos x)(1)

=

x2

-x sin x - cos x

=

x2

sin x cos x

=- x

-

x2

d 16.

dx

sin x x

(cos x)(x) - (sin x)(1)

=

x2

x cos x - sin x

=

x2

cos x sin x = x - x2

d 17.

dx

x sin x

(1)(sin x) - (x)(cos x)

=

sin2 x

sin x - x cos x

=

sin2 x

d 18.

dx

x cos x

(1)(cos x) - (x)(- sin x)

=

cos2 x

cos x + x sin x

=

cos2 x

dy

du

19. = 6u

dx

dx

= 6(x2 + 1)(2x)

= 12x(x2 + 1)

20.

dy =

1

du

dx 2 u dx

1

=

(2x)

2 x2 - 1

2x =

2 x2 - 1 x

= x2 - 1

dy

du

21. = (cos u)

dx

dx

= 6 cos(6x)

dy

du

22. = (- sin u)

dx

dx

= (- sin(2x + 3)) (2)

= -2 sin(2x + 3)

dy 23. = 2 sin x cos x

dx

1

Exercise 5A

Solutions to A.J. Sadler's

24. dy = 3 sin2 x cos x dx

25. dy = (5 cos4 x)(- sin x) dx = -5 cos4 x sin x

d 42. (5 sin 3x) = 5(3 cos 3x)

dx = 15 cos 3x

d 43. (2 sin 3x + 3 cos 2x) = 6 cos 3x - 6 sin 2x

dx

dy 26. = (- sin 3x)(3)

dx = -3 sin 3x

44. d (sin5 x) = (5 sin4 x)(cos x) dx = 5 sin4 x cos x

dy 27. = (cos(3x - 7)) (3)

dx = 3 cos(3x - 7)

45. d (5 cos2 x) = (10 cos x)(- sin x) dx = -10 cos x sin x

28.

dy =

cos(x2 - 3)

(2x)

dx

= 2x cos(x2 - 3)

46. d (-3 cos4 x) = (-12 cos3 x)(- sin x) dx = 12 cos3 x sin x

dy 29. = -3(- sin x)

dx = 3 sin x

dy 30. = 3 + 2(- sin x)

dx = 3 - 2 sin x

dy 31. = 2 cos 2x

dx (By this stage you should be getting to the point

47. d (cos0.5 x) = (0.5 cos-0.5 x)(- sin x) dx = -0.5 cos-0.5 x sin x

sin x = - 2 cos0.5 x

48.

d

sin x

=

(0.5 sin-0.5 x)(cos x)

dx

= 0.5 sin-0.5 x cos x

cos x =

2 sin x

of being able to do these in a single step.)

49?57 You should be able to do these in a single step

d

2

32. dx tan 2x = cos2 2x

. . . no working required. 58. f (x) = 1 cos x + x(- sin x)

dy 33. = 2x + sin x

dx

d 1 + sin x (cos x)(x2) - (1 + sin x)(2x)

34. dx

x2

=

x4

x2 cos x - 2x - 2x sin x

=

x4

x cos x - 2 - 2 sin x

=

x3

= cos x - x sin x 59. f (x) = 2x cos x + x2(- sin x)

= 2x cos x - x2 sin x

60. f (x) = 2 sin x + 2x(cos x) = 2 sin x + 2x cos x

61. f (x) = (2 sin 3x)(3 cos 3x)

35. d tan2 x = (2 tan x)(1 + tan2 x) dx = 2 tan x + 2 tan3 x

dy 36. = 3 cos x + 2 sin x

dx dy 37. = -3 sin 3x dx dy 38. = -9 sin 9x dx

= 6 sin 3x cos 3x

62. f (x) = (3 cos2 2x)(-2 sin 2x) = -6 cos2 2x sin 2x

63. (a) L.H.S:

d

d1

sec x =

dx

dx cos x

= d cos-1 x dx

= (- cos-2 x)(- sin x)

d

3

39. dx tan 3x = cos2 3x

d

1

2

40.

(tan x dx

+

tan 2x)

=

cos2

x

+

cos2

2x

d 41. (3 cos 2x) = 3(-2 sin 2x)

dx = -6 sin 2x

sin x = cos2 x

1 sin x =

cos x cos x = sec x tan x

= R.H.S.

2

Unit 3C Specialist Mathematics

Exercise 5A

(b)

d

d1

csc x =

dx

dx sin x

= d sin-1 x dx

= (- sin-2 x)(cos x)

cos x = - sin2 x

1 cos x =-

sin x sin x

= - csc x cot x

= R.H.S.

(c)

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

sin2 x + cos2 x = - sin2 x

1 = - sin2 x

= - csc2 x

= R.H.S.

64. cos

dy

= cos x

dx

3

=

6

2

dy

65.

= -2 sin 2x

dx

-2 sin 2 ? = -2 sin

6

3

3

= -2 ?

2

=- 3

dy

66.

= 2 cos x cos x + 2 sin x(- sin x)

dx

= 2 cos2 x - 2 sin2 x

= 2 cos 2x

2 cos(2 ? 0) = 2 alternatively:

2 sin x cos x = sin 2x d sin 2x = 2 cos 2x dx

2 cos(2 ? 0) = 2

dy

67.

= 6 sin x cos x

dx

6 sin cos = 0

dy

68.

= cos x

dx

d2y d dx2 = dx cos x

= - sin x

dy

69.

= -5 sin 5x

dx

d2y d

dx2

=

(-5 sin 5x) dx

= -25 cos 5x

dy

70.

= 6 cos 2x

dx

d2y d

dx2

=

(6 cos 2x) dx

= -12 sin 2x

dy

71.

= cos x - sin x

dx

d2y d

dx2

=

(cos x - sin x) dx

= - sin x - cos x

72. dy = 3 sin2 x cos x dx

d2y dx2

=

d (3 sin2 x cos x) dx

= (6 sin x cos x)(cos x) + (3 sin2 x)(- sin x)

= 6 sin x cos2 x - 3 sin3 x

= 6 sin x(1 - sin2 x) - 3 sin3 x

= 6 sin x - 6 sin3 x - 3 sin3 x

= 6 sin x - 9 sin3 x

dy

73.

= -4 cos x sin x

dx

d2y d

dx2

=

(-4 cos x sin x) dx

= (4 sin x)(sin x) + (-4 cos x)(cos x)

= 4 sin2 x - 4 cos2 x

= -4(cos2 x - sin2 x)

= -4 cos 2x

dy

74.

= sin x + x cos x

dx

sin + cos = 1 + 0

22 2

=1

y - y1 = m(x - x1)

y - = 1(x - )

2

2

y=x

dy

75.

= 1 - 6 sin 2x

dx

1 - 6 sin(2 ? 0) = 1

y - y1 = m(x - x1) y - 3 = 1(x - 0)

y = x+3

3

Exercise 5A

Solutions to A.J. Sadler's

76.

dy = 3(1 + tan2 2x)(2)

dx

= 6 + 6 tan2 2x

6

+

6

tan2(2

?

)

=

6

+

6

tan2

8

4

= 6 + 6(12)

= 12

y - y1 = m(x - x1)

y - 3 = 12(x - ) 8 3

y = 12x - + 3 2

77. (a) f (x) = 2 cos 2x f (/6) = 2 cos(/3) =1

(b) f (x) = -4 sin 2x f (/6) = -4 sin(/3) = -2 3

78. (a) f (x) = 2 sin x cos x = sin 2x

f (/6) = sin(/3) 3

= 2

(b) f (x) = 2 cos 2x f (/6) = 2 cos(/3) =1

79. (a) f (x) = 12 sin2 x cos x f (2) = 12 sin2(2) cos(2) = -4.13 (2d.p.)

(b) f (x) = (24 sin x cos x)(cos x) + (12 sin2 x)(- sin x)

= 24 sin x cos2 x - 12 sin3 x = 24 sin x(1 - sin2 x) - 12 sin3 x = 24 sin x - 24 sin3 x - 12 sin3 x = 24 sin x - 36 sin3 x f (2) = 24 sin(2) - 36 sin3(2) = -5.24 (2d.p.)

80. Because our limits are defined in terms of radians, it is necessary to do a conversion when working in degrees:

sin x = sin

x

180

dy

= cos x

dx

180 180

converting back to degrees = cos x 180

81. The length of the rectangle as drawn is 20 cos and the breadth is 20 sin so the area is given by

A = (20 cos )(20 sin ) = 400 sin cos = 200(2 sin cos ) = 200 sin 2

dA = 400 cos 2

d

Set the derivative to zero to find the maximum:

400 cos 2 = 0

2 = 2

which implies that the diagonals are perpendicular, hence the rectangle is a square.

A = 200 sin

2 = 200cm2

For side length, just take the square root:

l = 200

= 10 2

1 82. A = (8)(10) sin 0.1t

2 = 40 sin 0.1t

dA = 4 cos 0.1t

dt

dA (a) = 4 cos(0.1 ? 1)

dt = 3.98cm2/s

dA (b) = 4 cos(0.1 ? 5)

dt = 3.51cm2/s

dA (c) = 4 cos(0.1 ? 10)

dt = 2.16cm2/s

dA (d) = 4 cos(0.1 ? 20)

dt = -1.66cm2/s

83. (a) The maximum value of x is 5 and occurs

when

3t

=

2

,

i.e.

t

=

6

.

(b) 5 sin 3t = 2.5

sin 3t = 0.5

5 13 3t = , ,

66 6 5 13 t= , , 18 18 18 dx (c) = 15 cos 3t dt

= 15 cos(3 ? 0.6)

= -3.4

4

Unit 3C Specialist Mathematics

d2x d

(d)

dt2

=

15 cos 3t dt

= -45 sin 3t

= -9(5 sin 3t)

= -9x

Miscellaneous Exercise 5

k = -9.

d 3 sin + 4 cos = 3 cos - 4 sin

d 3 cos - 4 sin = 0

84.

4 sin = 3 cos

3 tan =

4 = 0.64353 sin + 4 cos = 5 (This can be confirmed as a maximum, if necessary, either graphically or by evaluating points on either side, or using the second derivative test.)

Miscellaneous Exercise 5

1. Draw and shade a circle centred at 3 + i having radius 3.

2. Proof by exhaustion: Because (-n)2 = n2 it will be sufficient to prove this for non-negative integers. Any non-negatve integer can be represented as n = 10t + u where t is a natural number and u is a single digit. Hence any square can be represented as

n2 = (10t + u)2 = 100t2 + 20ut + u2 = 10(10t2 + 2ut) + u2

Because 10(10t2 + 2ut) is a multiple of 10, it has a zero in the units digit, so the units digit of n2 is determined solely by the units digit of u2.

The possible units digit of any square number can hence be determined exhaustively:

u0123456789 u2 0 1 4 9 6 5 6 9 4 1

(where the tens digit of u2 has been discarded).

Thus the only possible last digits of any square number are 0, 1, 4, 5, 6 and 9. It is not possible to obtain a last digit of 2, 3, 7 or 8.

3.

r = (- 3)2 + 12

=2

1 tan =

-3

5

= (second quadrant)

6

5

(- 3 + i) = 2 cis

6

5

4. (a) sin x = cos x 3

tan x = 1 3

7 x= ,

66

(b) sin2 x + (1 + cos x)(cos x) = 0.5 sin2 x + cos x + cos2 x = 0.5 1 + cos x = 0.5 cos x = -0.5 2 x=? 3

(c) sin x(2 cos x - sin x) = cos2 x 2 sin x cos x - sin2 x = cos2 x sin 2x = sin2 x + cos2 x =1 3 2x = , - 22 3 x = ,- 44

dy (- sin x)(x) - (cos x)(1)

5. = dx

x2

x sin x + cos x

=-

x2

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