Trigonometry Examples

$\cos (- \frac{π}{8})$

Add full rotations of $2 π$ until the angle is greater than or equal to $0$ and less than $2 π$ .

$\cos (\frac{15 π}{8})$

The exact value of $\cos (\frac{15 π}{8})$ is $\frac{\sqrt{2 + \sqrt{2}}}{2}$ .

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Rewrite $\frac{15 π}{8}$ as an angle where the values of the six trigonometric functions are known divided by $2$ .

$\cos (\frac{\frac{15 π}{4}}{2})$

Apply the cosine half-angle identity $\cos (\frac{x}{2}) = \pm \sqrt{\frac{1 + \cos (x)}{2}}$ .

$\pm \sqrt{\frac{1 + \cos (\frac{15 π}{4})}{2}}$

Change the $\pm$ to $+$ because cosine is positive in the fourth quadrant.

$\sqrt{\frac{1 + \cos (\frac{15 π}{4})}{2}}$

Simplify $\sqrt{\frac{1 + \cos (\frac{15 π}{4})}{2}}$ .

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Subtract full rotations of $2 π$ until the angle is greater than or equal to $0$ and less than $2 π$ .

$\sqrt{\frac{1 + \cos (\frac{7 π}{4})}{2}}$

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant.

$\sqrt{\frac{1 + \cos (\frac{π}{4})}{2}}$

The exact value of $\cos (\frac{π}{4})$ is $\frac{\sqrt{2}}{2}$ .

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

Write $1$ as a fraction with a common denominator.

$\sqrt{\frac{\frac{2}{2} + \frac{\sqrt{2}}{2}}{2}}$

Combine the numerators over the common denominator.

$\sqrt{\frac{\frac{2 + \sqrt{2}}{2}}{2}}$

Multiply the numerator by the reciprocal of the denominator.

$\sqrt{\frac{2 + \sqrt{2}}{2} \cdot \frac{1}{2}}$

Multiply $\frac{2 + \sqrt{2}}{2} \cdot \frac{1}{2}$ .

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Multiply $\frac{2 + \sqrt{2}}{2}$ by $\frac{1}{2}$ .

$\sqrt{\frac{2 + \sqrt{2}}{2 \cdot 2}}$

Multiply $2$ by $2$ .

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

Rewrite $\sqrt{\frac{2 + \sqrt{2}}{4}}$ as $\frac{\sqrt{2 + \sqrt{2}}}{\sqrt{4}}$ .

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

Simplify the denominator.

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Rewrite $4$ as $2^{2}$ .

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

Pull terms out from under the radical, assuming positive real numbers.

$\frac{\sqrt{2 + \sqrt{2}}}{2}$

The result can be shown in multiple forms.

Exact Form:

$\frac{\sqrt{2 + \sqrt{2}}}{2}$

Decimal Form:

$0.92387953 \dots$