
Concept explainers
(a)
Find the symmetry of the points
(x1 , y1 ) and (x2, y2 ) .
(a)

Answer to Problem 84E
Rotated 180 deg counter clockwise about the origin.
Explanation of Solution
Given:Draw two points (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π+t1 respectively.
Concept Used:
Mathematically, symmetrymeans that one shape becomes exactly like another when you move it in some way: turn, flip or slide. For two objects to be symmetrical, they must be the same size and shape, with one object having a different orientation from the first. There can also be symmetry in one object, such as a face. If you draw a line of symmetry down the centre of your face, you can see that the left side is a mirror image of the right side. Not all objects have symmetry; if an object is not symmetrical, it is called asymmetric
Calculation:
The graph of the points shown below:![]() |
Let (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π+t1 respectively. x1 = cos t1 ; y1 = sin t1x2 = cos (π + t1)1 ; y2 = sin (π + t1) And the both points are rotated 180 deg counter clockwise about the origin. Therefore they are related as( x1 , y1 ) = ( −x2 , −y2 ) . Hence the points are rotated 180 deg counter clockwise about the origin. |
Thus, the points arerotated 180 deg counter clockwise about the origin.
(b)
Make a conjecture about any relationship between sin t1and sin (π + t1)
(b)

Answer to Problem 84E
sin t1 = − sin (π + t1)
Explanation of Solution
Given: Draw two points (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π+t1 respectively.
Concept Used:
The line (or "axis") of symmetry is the y-axis, also known as the line x=0. This line is marked green in the picture. The graph is said to be "symmetric about the y-axis", and this line of symmetry is also called the "axis of symmetry"
Calculation:
The graph of the points shown below:![]() |
Draw two points (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π − t1 respectively. x1 = cos t1 ; y1 = sin t1x2 = cos (π + t1)1 ; y2 = sin (π + t1) And the both points are rotated 180 deg counter clockwise about the origin. Therefore they are related as( x1 , y1 ) = ( −x2 , −y2 ) . Now we can see the points (x1 , y1 ) and (x2, y2 ) are rotated 180 deg counter clockwise about the origin. Since the graph has a rotational symmetry180 deg counter clockwise about the origin. which means sin t1 = − sin (π + t1) |
Thus, sin t1 = − sin (π + t1)
Make a conjecture about any relationship between cos t1and cos (π + t1)

Answer to Problem 84E
cos t1 = −cos (π + t1)
Explanation of Solution
Given: Draw two points (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π+t1 respectively.
Concept Used:The line (or "axis") of symmetry is the y-axis, also known as the line x=0. This line is marked green in the picture. The graph is said to be "symmetric about the y-axis", and this line of symmetry is also called the "axis of symmetry"
Calculation:
The graph of the points shown below:![]() |
Draw two points (x1 , y1 ) and (x2, y2 ) on the unit circle corresponding to t = t1 and t = π − t1 respectively. x1 = cos t1 ; y1 = sin t1x2 = cos (π + t11) ; y2 = sin (π + t1) And the both points are rotated 180 deg counter clockwise about the origin. Therefore they are related as( x1 , y1 ) = ( −x2 , −y2 ) . Now we can see the points (x1 , y1 ) and (x2, y2 ) are rotated 180 deg counter clockwise about the origin. Since the graph has a rotational symmetry 180 deg counter clockwise about the origin. which means cos t1 = −cos (π + t1) |
Thus, cos t1 = −cos (π + t1)
Chapter 4 Solutions
Precalculus with Limits: A Graphing Approach
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