# Trigonometric Identities and the Unit Circle - Questions With Detailed Solutions

How to use the unit circle to find properties and identities of the sine and cosine functions? Grade 11 trigonometry questions are presented along with detailed Solutions and explanations.

A circle has an infinite number of symmetries with respects to lines through the center and a symmetry with respect to its center. We are interested here on the symmetries with respect to its center, the x-axis, the y-axis an the line y = x. It will be shown how the use of these symmetries allows us to write several identities in trigonometry.

 Identities due to Symmetry of the Unit Circle on the origin, x and y axes Four angles (θ, π - θ, π + θ and 2π - θ) are shown below in a unit circle. To each angle corresponds a point (A, B, C or D) on the unit circle. . The four angles have the same reference angle equal to θ. Because of the symmetry of the circle, the four points form a rectangle ABCD as shown above. Points A and B are reflection of each other of the y-axis. Points A and C are reflection of each other on the origin of the system of axis. Points A and D are reflection of each other on the x-axis. Given the coordinates a and b of point A and using the symmetries of the circle, the coordinates of A, B, C and D are given by: A: (a , b) , B: (- a , b), C: (- a , - b) and D: (a , - b) We now express the coordinates of each point in terms of the sine and cosine of the corresponding angle as follows. A: (a , b) = (cos θ , sin θ) B: (- a , b) = (cos(π - θ) , sin(π - θ)) C: (- a , - b) = (cos(π + θ) , sin(π + θ)) D: (a , - b) = (cos(2π - θ) , sin(2π - θ)) Examples of Identities Comparing the x and y-coordinates of points A and B, we can write cos(π - θ) = - cos θ sin(π - θ) = sin θ Comparing the x and y-coordinates of points A and C, we can write cos(π + θ) = - cos θ sin(π + θ) = - sin θ Comparing the x and y-coordinates of points A and D, we can write cos(2π - θ) = cos θ sin(2π - θ) = - sin θ More Identities due to Symmetry of the Unit Circle on the x axis (Negative angles) Two angles θ, and - θ are shown below in a unit circle to which correspond the points A and D on the unit circle. . Points A and D are reflection of each other on the x-axis. Given the coordinates a and b of point A, the coordinates of D are given by: D: (a , - b) We now express the coordinates of points A and D in terms of the sine and cosine of the corresponding angle as follows. A: (a , b) = (cos θ , sin θ) D: (a , - b) = (cos(- θ) , sin(- θ)) Examples of Identities that may be Deduced cos(- θ) = cos θ sin( - θ) = - sin θ Identities due to Symmetry of the Unit Circle on the line y = x Points A and B shown in the unit circle below are reflection of each other on the line y = x. Because of the symmetry of the unit circle with respect to the line y = x, the corresponding angles to these points are θ and π/2 - θ as shown below. . Points A and B are reflection of each other on the line y = x. Given the coordinates a and b of point A, the coordinates of B are given by: B: (b , a) We now express the coordinates of points A and B in terms of the sine and cosine of the corresponding angle as follows. A: (a , b) = (cos θ , sin θ) B: (b , a) = (cos(π/2 - θ) , sin(π/2 - θ)) Examples of Identities that may be Deduced cos(π/2 - θ) = sin θ sin(π/2 - θ) = cos θ Use the following general identities 1) cos (A + B) = cos A cos B - sin A sin B 2) cos (A - B) = cos A cos B + sin A sin B 3) sin(A + B) = sin A cos B + cos A sin B 4) sin(A - B) = sin A cos B - cos A sin B to verify the identities found above and listed below. Detailed Solutions and explanations to these questions. cos(π - θ) = - cos θ sin(π - θ) = sin θ cos(π + θ) = - cos θ sin(π + θ) = - sin θ cos(2π - θ) = cos θ sin(2π - θ) = - sin θ cos(- θ) = cos θ sin( - θ) = - sin θ cos(π/2 - θ) = sin θ sin(π/2 - θ) = cos θ Detailed Solutions and explanations to these questions.

Updated: 9 March 2017 (A Dendane)