Find the Cosine Given the Point ((2 square root of 5)/5,-( square root of 5)/5)

$(\frac{2 \sqrt{5}}{5}, - \frac{\sqrt{5}}{5})$

To find the $\cos (θ)$ between the x-axis and the line between the points $(0, 0)$ and $(\frac{2 \sqrt{5}}{5}, - \frac{\sqrt{5}}{5})$ , draw the triangle between the three points $(0, 0)$ , $(\frac{2 \sqrt{5}}{5}, 0)$ , and $(\frac{2 \sqrt{5}}{5}, - \frac{\sqrt{5}}{5})$ .

Opposite : $- \frac{\sqrt{5}}{5}$

Adjacent : $\frac{2 \sqrt{5}}{5}$

Find the hypotenuse using Pythagorean theorem $c = \sqrt{a^{2} + b^{2}}$ .

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Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

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Apply the product rule to $\frac{2 \sqrt{5}}{5}$ .

$\sqrt{\frac{{(2 \sqrt{5})}^{2}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Apply the product rule to $2 \sqrt{5}$ .

$\sqrt{\frac{2^{2} {\sqrt{5}}^{2}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Simplify the numerator.

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Raise $2$ to the power of $2$ .

$\sqrt{\frac{4 {\sqrt{5}}^{2}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Rewrite ${\sqrt{5}}^{2}$ as $5$ .

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Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{5}$ as $5^{\frac{1}{2}}$ .

$\sqrt{\frac{4 {(5^{\frac{1}{2}})}^{2}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$\sqrt{\frac{4 \cdot 5^{\frac{1}{2} \cdot 2}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Combine $\frac{1}{2}$ and $2$ .

$\sqrt{\frac{4 \cdot 5^{\frac{2}{2}}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Cancel the common factor of $2$ .

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Cancel the common factor.

$\sqrt{\frac{4 \cdot 5^{\frac{2}{2}}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Rewrite the expression.

$\sqrt{\frac{4 \cdot 5^{1}}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Evaluate the exponent.

$\sqrt{\frac{4 \cdot 5}{5^{2}} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Reduce the expression by cancelling the common factors.

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Raise $5$ to the power of $2$ .

$\sqrt{\frac{4 \cdot 5}{25} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Multiply $4$ by $5$ .

$\sqrt{\frac{20}{25} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Cancel the common factor of $20$ and $25$ .

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Factor $5$ out of $20$ .

$\sqrt{\frac{5 (4)}{25} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Cancel the common factors.

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Factor $5$ out of $25$ .

$\sqrt{\frac{5 \cdot 4}{5 \cdot 5} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Cancel the common factor.

$\sqrt{\frac{5 \cdot 4}{5 \cdot 5} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Rewrite the expression.

$\sqrt{\frac{4}{5} + {(- \frac{\sqrt{5}}{5})}^{2}}$

Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

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Apply the product rule to $- \frac{\sqrt{5}}{5}$ .

$\sqrt{\frac{4}{5} + {(- 1)}^{2} {(\frac{\sqrt{5}}{5})}^{2}}$

Apply the product rule to $\frac{\sqrt{5}}{5}$ .

$\sqrt{\frac{4}{5} + {(- 1)}^{2} \frac{{\sqrt{5}}^{2}}{5^{2}}}$

Simplify the expression.

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Raise $- 1$ to the power of $2$ .

$\sqrt{\frac{4}{5} + 1 \frac{{\sqrt{5}}^{2}}{5^{2}}}$

Multiply $\frac{{\sqrt{5}}^{2}}{5^{2}}$ by $1$ .

$\sqrt{\frac{4}{5} + \frac{{\sqrt{5}}^{2}}{5^{2}}}$

Rewrite ${\sqrt{5}}^{2}$ as $5$ .

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Use $\sqrt[n]{a^{x}} = a^{\frac{x}{n}}$ to rewrite $\sqrt{5}$ as $5^{\frac{1}{2}}$ .

$\sqrt{\frac{4}{5} + \frac{{(5^{\frac{1}{2}})}^{2}}{5^{2}}}$

Apply the power rule and multiply exponents, ${(a^{m})}^{n} = a^{m n}$ .

$\sqrt{\frac{4}{5} + \frac{5^{\frac{1}{2} \cdot 2}}{5^{2}}}$

Combine $\frac{1}{2}$ and $2$ .

$\sqrt{\frac{4}{5} + \frac{5^{\frac{2}{2}}}{5^{2}}}$

Cancel the common factor of $2$ .

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Cancel the common factor.

$\sqrt{\frac{4}{5} + \frac{5^{\frac{2}{2}}}{5^{2}}}$

Rewrite the expression.

$\sqrt{\frac{4}{5} + \frac{5^{1}}{5^{2}}}$

Evaluate the exponent.

$\sqrt{\frac{4}{5} + \frac{5}{5^{2}}}$

Raise $5$ to the power of $2$ .

$\sqrt{\frac{4}{5} + \frac{5}{25}}$

Cancel the common factor of $5$ and $25$ .

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Factor $5$ out of $5$ .

$\sqrt{\frac{4}{5} + \frac{5 (1)}{25}}$

Cancel the common factors.

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Factor $5$ out of $25$ .

$\sqrt{\frac{4}{5} + \frac{5 \cdot 1}{5 \cdot 5}}$

Cancel the common factor.

$\sqrt{\frac{4}{5} + \frac{5 \cdot 1}{5 \cdot 5}}$

Rewrite the expression.

$\sqrt{\frac{4}{5} + \frac{1}{5}}$

Combine the numerators over the common denominator.

$\sqrt{\frac{4 + 1}{5}}$

Add $4$ and $1$ .

$\sqrt{\frac{5}{5}}$

Divide $5$ by $5$ .

$\sqrt{1}$

Any root of $1$ is $1$ .

$1$

$\cos (θ) = \frac{Adjacent}{Hypotenuse}$ therefore $\cos (θ) = \frac{\frac{2 \sqrt{5}}{5}}{1}$ .

$\frac{\frac{2 \sqrt{5}}{5}}{1}$

Divide $\frac{2 \sqrt{5}}{5}$ by $1$ .

$\cos (θ) = \frac{2 \sqrt{5}}{5}$

Approximate the result.

$\cos (θ) = \frac{2 \sqrt{5}}{5} \approx 0.89442719$

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