5.5.1 Proving Trigonometric Identities Using Addition Formula And Double Angle Formulae (Part 1)


Example 2:
Prove each of the following trigonometric identities.
(a)1+cos2xsin2x=cotx(b)cotAsec2A=cotA+tan2A(c)sinx1cosx=cotx2

Solution:
(a)
LHS=1+cos2xsin2x=1+(2cos2x1)2sinxcosx=2cos2x2sinxcosx=cosxsinx=cotx=RHS(proven)


(b)
RHS=cotA+tan2A=cosAsinA+sin2Acos2A=cosAcos2A+sinAsin2AsinAcos2A=cosA(cos2Asin2A)+sinA(2sinAcosA)sinAcos2A=cos3AcosAsin2A+2sin2AcosAsinAcos2A=cos3A+cosAsin2AsinAcos2A=cosA(cos2A+sin2A)sinAcos2A=cosAsinAcos2Asin2A+cos2A=1=(cosAsinA)(1cos2A)=cotAsec2A


(c)
LHS=sinx1cosx=2sinx2cosx21(12sin2x2)sinx=2sinx2cosx2,cosx=12sin2x2=2sinx2cosx22sin2x2=cosx2sinx2=cotx2=RHS(proven)

5.1 Positive and Negative Angles

5.1 Positive and Negative Angles
1. Positive angles are angles measure in an anticlockwise rotate from the positive x-axis about the origin, O.

2. Negative angles are angles measured in a clockwise rotation from the positive x-axis about the origin O.


3. One complete revolution is 360° or 2π radians.


Example:
Show each of the following angles on a separate diagram and state the quadrant in which the angle is situated.
(a) 410°
(b) 890°
(c)229π radians  
(d)103π radians
(e) –60o
(f) –500°
(g)314π radians

Solution:
(a)
410° = 360° + 50°
Based on the above circular diagram, the positive angle of 410° is in the first quadrant.


(b)
890° = 720° + 170°
Based on the above circular diagram, the positive angle of 890° is in the second quadrant.


(c)

229πrad=(2π+49π)rad=360o+80o
Based on the above circular diagram, the positive angle of 229π radians  is in the first quadrant.


(d)
103πrad=(3π+13π)rad=540o+60o
Based on the above circular diagram, the positive angle of 103π radians  is in the third quadrant.



(e)
Based on the above circular diagram, the negative angle of –60° is in the fourth quadrant.



(f)
–500° = –360° – 140°
Based on the above circular diagram, the negative angle of –500° is in the third quadrant.


(g)

314πrad=(3π14π)rad=540o45o
Based on the above circular diagram, the negative angle of 314π radians  is in the second quadrant.


5.2.2 Six Trigonometric Functions of Any Angle

5.2b Six Trigonometric Functions of Any Angle 

(B) Special Angles

(1) Value of Special Angle 30° and 60°
 
  (a)sin30o=12 (b)cos30o=32(c)tan30o=13  (d)sin60o=32  (e)cos60o=12   (f)tan60o=3 


(2) Value of Special Angle 45°
 
 
  (a)sin45o=12  (b)cos45o=12  (c)tan45o=1     


(3) Value of Special Angle 0°, 90°, 180°, 270°, 360°
 
(a) y = sin x
 



x
0o
90o
180o
270o
360o
sin
0
1
0
-1
0

(b) y = cos x





(c) y = tan x
 


x
0o
90o
180o
270o
360o
tan 
0
  ∞
0
  ∞
0

Short Question 4 – 6


Question 4:
Solve the equation 3 sin A cos A – cos A = 0 for 0°  A  360°.

Solution:
3 sin A cos A – cos A = 0
cos A (3 sin A – 1) = 0
cos A = 0   or   sin A

cos A = 0
A = 90°, 270°

sin A
Basic angle = 19°28'
A = 19°28', 180° – 19°28'
A = 19°28', 160°32'

Hence A = 19°28', 90°, 160°32', 270°.



Question 5:
Solve the equation 4 sin (x – π) cos (x – π) = 1 for 0ox ≤ 360o.
 
Solution:
4 sin (x – π) cos (x – π) = 1
2 [2 sin (x – π) cos (x – π)] = 1
2 sin (x – π) cos (x – π) = ½
sin 2(x – π) = ½  ← (sin 2x= 2 sinx cosx)
sin 2(x – 180o) = ½  ← (π rad = 180o)
sin (2x – 360o) = ½
sin 2x cos 360o – cos 2x sin 360o = ½
sin 2x (1) – cos 2x (0)  = ½  ← (cos 360o = 1, sin 360o = 0)
sin 2x = ½
basic angle = 30o  ← (special angle, sin 30o= ½)
2x = 30o, 150o, 390o, 510o
x = 15o, 75o, 195o, 255o

Long Question 1 & 2


Question 1:
(a) Sketch the graph of y = cos 2x for 0°  x  180°.
(b) Hence, by drawing a suitable straight line on the same axes, find the number of solutions satisfying the equation 2 sin2x=2x180 for 0°  x  180°.

Solution:

(a)(b)

2 sin2x=2x18012 sin2x=1(2x180)cos2x=x1801y=x1801x=0,  y=1x=180,  y=0Number of solutions = 2



Question 2:
(a) Sketch the graph of y=32cos2x for 0x32π.
(b) Hence, using the same axes, sketch a suitable straight line to find the number of solutions to the equation 43πxcos2x=32 for 0x32π
State the number of solutions.

Solution:

(a)(b)



43πxcos2x=32cos2x=43πx3232cos2x=32(43πx32)y=2πx94To sketch the graph of y=2πx94x=0, y=94x=3π2, y=34Number of solutions =Number of intersection points= 3

5.5 Formulae of sin (A ± B), cos (A ± B), tan (A ± B), sin 2A, cos 2A, tan 2A


5.5 Formulae of sin (A ± B), cos (A ± B), tan (A ± B), sin 2A, cos 2A, tan 2A

(A) Compound Angles Formulae:
sin(A±B)=sinAcosB±cosAsinBcos(A±B)=cosAcosBsinAsinBtan(A±B)=tanA±tanB1tanAtanB


(B) Double Angle Formulae:

  • sin 2A = 2 sin A cos A
  • cos 2A = cos2 A – sin2 A
  • cos 2A = 2 cos2 A – 1
  • cos 2A = 1 – 2 sin2 A
  • tan2A=2tanA1tan2A   

(C) Half Angle Formulae:
   sinA=2sinA2cosA2   cosA=sin2A2cos2A2    cosA=2cos2A21  cosA=12cos2A2   tanA=2tanA21tan2A2


5.5.1 Proving Trigonometric Identities using Addition Formula and Double Angle Formulae

Example 1:
Prove each of the following trigonometric identities.
(a) sin(A+B)sin(AB)cosAcosB=2tanB(b) cos(A+B)sinAcosB=cotAtanB(c) tan(A+45o)=sinA+cosAcosAsinA

Solution:
(a)
LHS=sin(A+B)sin(AB)cosAcosB=(sinAcosB+cosAsinB)(sinAcosBcosAsinB)cosAcosB=2cosAsinBcosAcosB=2sinBcosB=2tanB=RHS (proven)


(b)
LHS=cos(A+B)sinAcosB=cosAcosBsinAsinBsinAcosB=cosAcosBsinAcosBsinAsinBsinAcosB=cosAsinAsinBcosB=cotAtanB=RHS(proven)  


(c)
LHS=tan(A+45o)=tanA+tan45o1tanAtan45o=tanA+11tanAtan45o=1=sinAcosA+11sinAcosA=sinA+cosAcosA×cosAcosAsinA=sinA+cosAcosAsinA=RHS(proven)

Short Question 15 – 18


Question 15:
Prove the identity2cos2A+1=sec2A

Solution:
LHS=2cos2A+1=2(2cos2A1)+1cos2A=2cos2A1=22cos2A=1cos2A=sec2A=RHSProven
 


Question 16:
Prove the identity2tanA2sec2A=tan2A

Solution:
LHS=2tanA2sec2A=2tanA2(tan2A+1)tan2A+1=sec2A=2tanA1tan2A=tan2A=RHSProven



Question 17:
Prove the identitytanx+cotx=2cosec2x

Solution:
LHS=tanx+cotx=sinxcosx+cosxsinx=sin2x+cos2xcosxsinx=1cosxsinxsin2x+cos2x=1=112sin2xsin2x=2sinxcosx12sin2x=sinxcosx=2sin2x=2(1sin2x)=2cosec2x=RHSProven
 


Question 18:
Prove the identitycosxsin2xcos2x+sinx1=1tanx

Solution:
LHS=cosxsin2xcos2x+sinx1=cosx2sinxcosx(12sin2x)+sinx1cos2x=12sin2x=cosx(12sinx)sinx2sin2x=cosx(12sinx)sinx(12sinx)=cosxsinx=cotx=1tanx=RHSProven

5.2.1 Six Trigonometric Functions of Any Angle


5.2a Six Trigonometric Functions of Any Angle 

(A) The definition of sin, cos, tan, cosec, sec and cot


1. Let P (x, y) be any point on the circumference of the circle with centre O and of radius r. Based on ∆ OPQ in the above diagram,



2. The definitions of tangent, cotangent, secant and cosecant of any angle are:





3. The relations of the trigonometric ratio of an angle θ with its complementary angle (90oθ) are:


For examples:
(a) sin 75= cos (90o – 75o) = cos 15o
(b) tan 50= cot (90o – 50o) = cot 40o
(c) sec 25o= cosec (90o – 25o) = cosec 65o



4. The trigonometric ratios of any negative angle (–θ) are:



  • A negative angle is an angle measured in a clockwise direction from the positive x-axis.
  • For example, – 60ois equivalent to 300o (360o – 60o).

Example:
Express each of the following trigonometric functions in terms of the trigonometric ratios of acute angles. Hence, find each value using a calculator.
(a) cos (– 325o)
(b) tan (– 124o)
(c) sin (– 115o)

Solution:
(a)
cos (– 325o)
= cos 325o  ← {The formula cos (–θ) = cos θ is used}
= cos (360o– 325o)  ← {At fourth quadrant, cos is positive}
= cos 35o
= 0.8192

(b) 
tan (– 124o)
= – tan 124o   ← {The formula tan (–θ) = – tan θ is used}
= – [– tan (180o– 124o)]  ← {At second quadrant, tan is negative}
= tan 56o
= 1.483

(c)
sin (– 115o)
= – sin 115o   ← {The formula sin (–θ) = – sin θ is used}
= – sin (180o– 115o)  ← {At second quadrant, sin is positive}
= – sin 65o
= – 0.9063

5.6a Solving Trigonometric Equation (Basic Equation In Sinx/ Cosx/ Tanx/ Cosecx/ Secx/ Cotx)


5.6 Simple Trigonometric Equations

Steps to solve simple trigonometric equation:
(1) Determine the range of values of the required angles.
(2) Find a basic angle by using calculator.
(3) Determine the quadrants the angle should be.
(4) Determine the values of angles in those quadrants.



(A) Solving Trigonometric Equation (Basic Equation in sinx/ cosx/ tanx/ cosecx/ secx/ cotx)

Example:
Find all the values of θ  for 0° < θ  < 360° that satisfy each of the following trigonometric equations.
(a) sin θ = 0.6137
(b) cos θ = 0.2377
(c) tan θ = 2.7825
(d) sin θ = -0.8537
(e) sin 2θ = 0.5293

Solution:
(a)
sin 
θ = 0.6137
basic angle = sinˉ¹ 0.6137 = 37.86°
θ = 37.86°, 180°-37.86°
θ = 37.86°, 142.14°

(b)
cos θ  = 0.2377
basic angle = cosˉ¹ 0.2377 = 76.25°
θ = 76.25°, 360° – 76.25°
θ = 76.25°, 283.75°

(c)
tan θ  = 2.7825
basic angle = tanˉ¹ 2.7825 = 70.23°
θ = 70.23°, 180° + 70.23°
θ = 70.23°, 250.23°

(d)
sin 
θ = -0.8537
basic angle = sinˉ¹ 0.8537 = 58.62°
θ = 180° + 58.62°, 360° – 58.62°
θ = 238.62°, 301.38°
 

(e)
sin 2
θ = 0.5293
basic angle = 31.96°
0° < θ  < 360°
0° < 2θ  < 720°
2θ = 31.96°, 180° – 31.96°, 360° + 31.96°, 360° + 180° – 31.96°
2θ = 31.96°, 148.04°, 391.96°, 508.04°
θ = 15.98°, 74.02°, 195.98°, 254.02°

Short Question 9 & 10


Question 9:
Given that sin θ = 35 , where θ is an acute angle, without using tables or a calculator, find the values of
(a) sin (180º + θ),
(b) cos (180º – θ),
(c) tan (360º + θ).

Solution:
(a)


sinθ=35cosθ=45tanθ=34

sin (180º + θ)
= sin 180º cos θ + cos 180º sin θ
= (0) cos θ + (– 1) sin θ
= – sin θ
= 35

(b)
cos (180º – θ)
= cos 180º cos θ + sin 180º sin θ
= (– 1) cos θ + (0) sin θ
= – cos θ
45

(c)
tan(360+θ)=tan360+tanθ1tan360tanθ=0+tanθ1(0)(tanθ)=tanθ=34



Question 10:
Prove each of the following trigonometric identities.
(a) cot2 x – cot2 x cos2x = cos2 x
(b)secxsecxcosx=cosec2x

Solution:
(a)
LHS:cot2xcot2xcos2x=cot2x(1cos2x)=cot2x(sin2x)=cos2xsin2x(sin2x)=cos2x(RHS)


(b)
LHS:secxsecxcosx=1cosx1cosxcosx=1cosx1cosxcos2xcosx=1cosx1cos2xcosx=1cosx×cosx1cos2x=11cos2x=1sin2x=cosec2x(RHS)