Find the Exact Value 2cos(112.5 degrees )^2-1

$2 \cos^{2} (112.5 °) - 1$

Step 1

Apply the cosine double-angle identity.

$\cos (2 \cdot 112.5 °)$

Step 2

Multiply $2$ by $112.5 °$ .

$\cos (225)$

Step 3

The exact value of $\cos (225)$ is $- \frac{\sqrt{2}}{2}$ .

Rewrite $225$ as an angle where the values of the six trigonometric functions are known divided by $2$ .

$\cos (\frac{450}{2})$

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

$\pm \sqrt{\frac{1 + \cos (450)}{2}}$

Change the $\pm$ to $-$ because cosine is negative in the third quadrant.

$- \sqrt{\frac{1 + \cos (450)}{2}}$

Simplify $- \sqrt{\frac{1 + \cos (450)}{2}}$ .

Remove full rotations of $360$ ° until the angle is between $0$ ° and $360$ °.

$- \sqrt{\frac{1 + \cos (90)}{2}}$

The exact value of $\cos (90)$ is $0$ .

$- \sqrt{\frac{1 + 0}{2}}$

Add $1$ and $0$ .

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

Rewrite $\sqrt{\frac{1}{2}}$ as $\frac{\sqrt{1}}{\sqrt{2}}$ .

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

Any root of $1$ is $1$ .

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

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

$- (\frac{1}{\sqrt{2}} \cdot \frac{\sqrt{2}}{\sqrt{2}})$

Combine and simplify the denominator.

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

$- \frac{\sqrt{2}}{\sqrt{2} \sqrt{2}}$

Raise $\sqrt{2}$ to the power of $1$ .

$- \frac{\sqrt{2}}{{\sqrt{2}}^{1} \sqrt{2}}$

Raise $\sqrt{2}$ to the power of $1$ .

$- \frac{\sqrt{2}}{{\sqrt{2}}^{1} {\sqrt{2}}^{1}}$

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

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

Add $1$ and $1$ .

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

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

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

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

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

$- \frac{\sqrt{2}}{2^{\frac{1}{2} \cdot 2}}$

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

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

Cancel the common factor of $2$ .

Cancel the common factor.

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

Rewrite the expression.

$- \frac{\sqrt{2}}{2^{1}}$

Evaluate the exponent.

$- \frac{\sqrt{2}}{2}$

Step 4

The result can be shown in multiple forms.

Exact Form:

$- \frac{\sqrt{2}}{2}$

Decimal Form:

$- 0.70710678 \dots$

Find the Exact Value 2cos(112.5 degrees )^2-1

Related Questions

Support LCMGCF.com by disabling your adblocker.