The Unit Circle: Tutorial
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Complete the statement. In your answer, rationalize the denominator and write it in simplest form.

If [tex]\tan (\theta) = \frac{3}{5}[/tex], and [tex]\theta[/tex] is an angle in quadrant I, what is the value of [tex]\cos (\theta)[/tex]?

[tex]\cos (\theta) = [/tex]



Answer :

To determine the value of [tex]\(\cos(\theta)\)[/tex] given that [tex]\(\tan(\theta) = \frac{3}{5}\)[/tex] and that [tex]\(\theta\)[/tex] is an angle in the first quadrant, we will follow these steps:

1. Understand the relationship of [tex]\(\tan(\theta)\)[/tex], [tex]\(\sin(\theta)\)[/tex], and [tex]\(\cos(\theta)\)[/tex]:

[tex]\(\tan(\theta) = \frac{\sin(\theta)}{\cos(\theta)}\)[/tex]. Given [tex]\(\tan(\theta) = \frac{3}{5}\)[/tex], we can write:

[tex]\[ \frac{\sin(\theta)}{\cos(\theta)} = \frac{3}{5} \][/tex]

2. Express [tex]\(\sin(\theta)\)[/tex] in terms of [tex]\(\cos(\theta)\)[/tex]:

[tex]\[ \sin(\theta) = 3k \quad \text{and} \quad \cos(\theta) = 5k \][/tex]

Here, [tex]\(k\)[/tex] is a proportionality constant.

3. Use the Pythagorean identity [tex]\(\sin^2(\theta) + \cos^2(\theta) = 1\)[/tex]:

Substitute [tex]\(\sin(\theta)\)[/tex] and [tex]\(\cos(\theta)\)[/tex] into the Pythagorean identity:

[tex]\[ (3k)^2 + (5k)^2 = 1 \][/tex]

Simplify the equation:

[tex]\[ 9k^2 + 25k^2 = 1 \][/tex]

Combine the terms:

[tex]\[ 34k^2 = 1 \][/tex]

Solve for [tex]\(k^2\)[/tex]:

[tex]\[ k^2 = \frac{1}{34} \][/tex]

Therefore,

[tex]\[ k = \sqrt{\frac{1}{34}} = \frac{1}{\sqrt{34}} \][/tex]

4. Find [tex]\(\cos(\theta)\)[/tex]:

Recall that [tex]\(\cos(\theta) = 5k\)[/tex]:

[tex]\[ \cos(\theta) = 5 \cdot \frac{1}{\sqrt{34}} = \frac{5}{\sqrt{34}} \][/tex]

Rationalize the denominator by multiplying the numerator and the denominator by [tex]\(\sqrt{34}\)[/tex]:

[tex]\[ \cos(\theta) = \frac{5}{\sqrt{34}} \cdot \frac{\sqrt{34}}{\sqrt{34}} = \frac{5\sqrt{34}}{34} \][/tex]

Thus, the value of [tex]\(\cos(\theta)\)[/tex] when [tex]\(\tan(\theta) = \frac{3}{5}\)[/tex] in the first quadrant is:

[tex]\[ \cos(\theta) = \frac{5\sqrt{34}}{34} \][/tex]