
Concept explainers
a.
To verify that the expression for s is equivalent to hcotθ .
a.

Explanation of Solution
Given:
The length s of a shadow cast by a vertical gnomon of height h when the angle of the sun above the horizon is θ can be modeled by s=hsin(90°−θ)sinθ .
Concept used:
- Cofunction Identities: The value of any trigonometric function of an angle θ is equal to the cofunction of the complement of θ . The cofunction identities given by
- Quotient Identities:
sinθ=cos(π2−θ),sinθ=cos(π2−θ)tanθ=cot(π2−θ),cotθ=tan(π2−θ)secθ=csc(π2−θ),cscθ=sec(π2−θ)
tanθ=sinθcosθ,cotθ=cosθsinθ
Calculation:
In order to verify the expression for s , use the cofunction identities and simplify further as shown below,
s=hsin(90°−θ)sinθs=hcosθsinθ(sin(90°−θ)=cosθ)s=h⋅cosθsinθs=h⋅cotθs=hcotθ
Thus, it verifies that the expression for s is equivalent to hcotθ .
b.
To complete the table when h=5 .
b.

Answer to Problem 79E
θ | 15° | 30° | 45° | 60° | 75° | 90° |
s | 18.66 | 8.66 | 5 | 2.89 | 1.34 | 0 |
Explanation of Solution
Given:
The length s of a shadow cast by a vertical gnomon of height h when the angle of the sun above the horizon is θ can be modeled by s=hsin(90°−θ)sinθ .
Calculation:
Using a graphing utility, the value of the s at different angles shown below,
θ | 15° | 30° | 45° | 60° | 75° | 90° |
s | 18.66 | 8.66 | 5 | 2.89 | 1.34 | 0 |
c.
To verify that the expression for s is equivalent to hcotθ .
c.

Answer to Problem 79E
Maximum length at:15°Minimum length at:90°
Explanation of Solution
Given:
The length s of a shadow cast by a vertical gnomon of height h when the angle of the sun above the horizon is θ can be modeled by s=hsin(90°−θ)sinθ .
Calculation:
From the above table in part (b) it is clear that the maximum length of the shadow was at 15° , and the minimum length of the shadow was at 90° .
d.
The time of the day when the angle of the sun above the horizon is 90° .
d.

Answer to Problem 79E
Noon 12:00 PM
Explanation of Solution
Given:
The length s of a shadow cast by a vertical gnomon of height h when the angle of the sun above the horizon is θ can be modeled by s=hsin(90°−θ)sinθ .
Calculation:
Since the length of the shadow s was zero when the angle of the sun above the horizon was 90° . That means at that time the sun will be just at top of the gnomon. In a day the sub be at the top of head at the time of noon.
So, when the angle of the sun above the horizon is 90° , the time of the day would be approximately 12:00 PM noon.
Chapter 5 Solutions
Precalculus with Limits: A Graphing Approach
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