Evaluate cos(pi/4+pi/3)

$\cos (\frac{π}{4} + \frac{π}{3})$

Step 1

To write $\frac{π}{4}$ as a fraction with a common denominator, multiply by $\frac{3}{3}$ .

$\cos (\frac{π}{4} \cdot \frac{3}{3} + \frac{π}{3})$

Step 2

To write $\frac{π}{3}$ as a fraction with a common denominator, multiply by $\frac{4}{4}$ .

$\cos (\frac{π}{4} \cdot \frac{3}{3} + \frac{π}{3} \cdot \frac{4}{4})$

Step 3

Write each expression with a common denominator of $12$ , by multiplying each by an appropriate factor of $1$ .

Multiply $\frac{π}{4}$ by $\frac{3}{3}$ .

$\cos (\frac{π \cdot 3}{4 \cdot 3} + \frac{π}{3} \cdot \frac{4}{4})$

Multiply $4$ by $3$ .

$\cos (\frac{π \cdot 3}{12} + \frac{π}{3} \cdot \frac{4}{4})$

Multiply $\frac{π}{3}$ by $\frac{4}{4}$ .

$\cos (\frac{π \cdot 3}{12} + \frac{π \cdot 4}{3 \cdot 4})$

Multiply $3$ by $4$ .

$\cos (\frac{π \cdot 3}{12} + \frac{π \cdot 4}{12})$

Step 4

Combine the numerators over the common denominator.

$\cos (\frac{π \cdot 3 + π \cdot 4}{12})$

Step 5

Simplify the numerator.

Move $3$ to the left of $π$ .

$\cos (\frac{3 \cdot π + π \cdot 4}{12})$

Move $4$ to the left of $π$ .

$\cos (\frac{3 π + 4 \cdot π}{12})$

Add $3 π$ and $4 π$ .

$\cos (\frac{7 π}{12})$

Step 6

The exact value of $\cos (\frac{7 π}{12})$ is $- \frac{\sqrt{2 - \sqrt{3}}}{2}$ .

Rewrite $\frac{7 π}{12}$ as an angle where the values of the six trigonometric functions are known divided by $2$ .

$\cos (\frac{\frac{7 π}{6}}{2})$

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

$\pm \sqrt{\frac{1 + \cos (\frac{7 π}{6})}{2}}$

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

$- \sqrt{\frac{1 + \cos (\frac{7 π}{6})}{2}}$

Simplify $- \sqrt{\frac{1 + \cos (\frac{7 π}{6})}{2}}$ .

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the third quadrant.

$- \sqrt{\frac{1 - \cos (\frac{π}{6})}{2}}$

The exact value of $\cos (\frac{π}{6})$ is $\frac{\sqrt{3}}{2}$ .

$- \sqrt{\frac{1 - \frac{\sqrt{3}}{2}}{2}}$

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

$- \sqrt{\frac{\frac{2}{2} - \frac{\sqrt{3}}{2}}{2}}$

Combine the numerators over the common denominator.

$- \sqrt{\frac{\frac{2 - \sqrt{3}}{2}}{2}}$

Multiply the numerator by the reciprocal of the denominator.

$- \sqrt{\frac{2 - \sqrt{3}}{2} \cdot \frac{1}{2}}$

Multiply $\frac{2 - \sqrt{3}}{2} \cdot \frac{1}{2}$ .

Multiply $\frac{2 - \sqrt{3}}{2}$ by $\frac{1}{2}$ .

$- \sqrt{\frac{2 - \sqrt{3}}{2 \cdot 2}}$

Multiply $2$ by $2$ .

$- \sqrt{\frac{2 - \sqrt{3}}{4}}$

Rewrite $\sqrt{\frac{2 - \sqrt{3}}{4}}$ as $\frac{\sqrt{2 - \sqrt{3}}}{\sqrt{4}}$ .

$- \frac{\sqrt{2 - \sqrt{3}}}{\sqrt{4}}$

Simplify the denominator.

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

$- \frac{\sqrt{2 - \sqrt{3}}}{\sqrt{2^{2}}}$

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

$- \frac{\sqrt{2 - \sqrt{3}}}{2}$

Step 7

The result can be shown in multiple forms.

Exact Form:

$- \frac{\sqrt{2 - \sqrt{3}}}{2}$

Decimal Form:

$- 0.25881904 \dots$

Evaluate cos(pi/4+pi/3)

Related Questions

Support LCMGCF.com by disabling your adblocker.