Find the Exact Value cos((3pi)/4)cos((5pi)/12)-sin((3pi)/4)sin((5pi)/12)

$\cos (\frac{3 π}{4}) \cos (\frac{5 π}{12}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Step 1

Simplify each term.

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 second quadrant.

$- \cos (\frac{π}{4}) \cos (\frac{5 π}{12}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{2}}{2} \cos (\frac{5 π}{12}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

The exact value of $\cos (\frac{5 π}{12})$ is $\frac{\sqrt{6} - \sqrt{2}}{4}$ .

Split $\frac{5 π}{12}$ into two angles where the values of the six trigonometric functions are known.

$- \frac{\sqrt{2}}{2} \cos (\frac{π}{6} + \frac{π}{4}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Apply the sum of angles identity $\cos (x + y) = \cos (x) \cos (y) - \sin (x) \sin (y)$ .

$- \frac{\sqrt{2}}{2} (\cos (\frac{π}{6}) \cos (\frac{π}{4}) - \sin (\frac{π}{6}) \sin (\frac{π}{4})) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3}}{2} \cos (\frac{π}{4}) - \sin (\frac{π}{6}) \sin (\frac{π}{4})) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2} - \sin (\frac{π}{6}) \sin (\frac{π}{4})) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

The exact value of $\sin (\frac{π}{6})$ is $\frac{1}{2}$ .

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2} - \frac{1}{2} \sin (\frac{π}{4})) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2} - \frac{1}{2} \cdot \frac{\sqrt{2}}{2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

Simplify each term.

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

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

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3} \sqrt{2}}{2 \cdot 2} - \frac{1}{2} \cdot \frac{\sqrt{2}}{2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Combine using the product rule for radicals.

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{3 \cdot 2}}{2 \cdot 2} - \frac{1}{2} \cdot \frac{\sqrt{2}}{2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $3$ by $2$ .

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{6}}{2 \cdot 2} - \frac{1}{2} \cdot \frac{\sqrt{2}}{2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $2$ by $2$ .

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{6}}{4} - \frac{1}{2} \cdot \frac{\sqrt{2}}{2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

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

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{6}}{4} - \frac{\sqrt{2}}{2 \cdot 2}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $2$ by $2$ .

$- \frac{\sqrt{2}}{2} (\frac{\sqrt{6}}{4} - \frac{\sqrt{2}}{4}) - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Combine the numerators over the common denominator.

$- \frac{\sqrt{2}}{2} \cdot \frac{\sqrt{6} - \sqrt{2}}{4} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $- \frac{\sqrt{2}}{2} \cdot \frac{\sqrt{6} - \sqrt{2}}{4}$ .

Multiply $\frac{\sqrt{6} - \sqrt{2}}{4}$ by $\frac{\sqrt{2}}{2}$ .

$- \frac{(\sqrt{6} - \sqrt{2}) \sqrt{2}}{4 \cdot 2} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $4$ by $2$ .

$- \frac{(\sqrt{6} - \sqrt{2}) \sqrt{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Apply the distributive property.

$- \frac{\sqrt{6} \sqrt{2} - \sqrt{2} \sqrt{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Combine using the product rule for radicals.

$- \frac{\sqrt{6 \cdot 2} - \sqrt{2} \sqrt{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $- \sqrt{2} \sqrt{2}$ .

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

$- \frac{\sqrt{6 \cdot 2} - ({\sqrt{2}}^{1} \sqrt{2})}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{6 \cdot 2} - ({\sqrt{2}}^{1} {\sqrt{2}}^{1})}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{6 \cdot 2} - {\sqrt{2}}^{1 + 1}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Add $1$ and $1$ .

$- \frac{\sqrt{6 \cdot 2} - {\sqrt{2}}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Simplify each term.

Multiply $6$ by $2$ .

$- \frac{\sqrt{12} - {\sqrt{2}}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Rewrite $12$ as $2^{2} \cdot 3$ .

Factor $4$ out of $12$ .

$- \frac{\sqrt{4 (3)} - {\sqrt{2}}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{2^{2} \cdot 3} - {\sqrt{2}}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Pull terms out from under the radical.

$- \frac{2 \sqrt{3} - {\sqrt{2}}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

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

$- \frac{2 \sqrt{3} - {(2^{\frac{1}{2}})}^{2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{2 \sqrt{3} - 2^{\frac{1}{2} \cdot 2}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{2 \sqrt{3} - 2^{\frac{2}{2}}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Cancel the common factor of $2$ .

Cancel the common factor.

$- \frac{2 \sqrt{3} - 2^{\frac{2}{2}}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Rewrite the expression.

$- \frac{2 \sqrt{3} - 2^{1}}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Evaluate the exponent.

$- \frac{2 \sqrt{3} - 1 \cdot 2}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Multiply $- 1$ by $2$ .

$- \frac{2 \sqrt{3} - 2}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Cancel the common factor of $2 \sqrt{3} - 2$ and $8$ .

Factor $2$ out of $2 \sqrt{3}$ .

$- \frac{2 (\sqrt{3}) - 2}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Factor $2$ out of $- 2$ .

$- \frac{2 (\sqrt{3}) + 2 (- 1)}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Factor $2$ out of $2 (\sqrt{3}) + 2 (- 1)$ .

$- \frac{2 (\sqrt{3} - 1)}{8} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Cancel the common factors.

Factor $2$ out of $8$ .

$- \frac{2 (\sqrt{3} - 1)}{2 (4)} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Cancel the common factor.

$- \frac{2 (\sqrt{3} - 1)}{2 \cdot 4} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

Rewrite the expression.

$- \frac{\sqrt{3} - 1}{4} - \sin (\frac{3 π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{3} - 1}{4} - \sin (\frac{π}{4}) \sin (\frac{5 π}{12})$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} \sin (\frac{5 π}{12})$

The exact value of $\sin (\frac{5 π}{12})$ is $\frac{\sqrt{2} + \sqrt{6}}{4}$ .

Split $\frac{5 π}{12}$ into two angles where the values of the six trigonometric functions are known.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} \sin (\frac{π}{6} + \frac{π}{4})$

Apply the sum of angles identity.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\sin (\frac{π}{6}) \cos (\frac{π}{4}) + \cos (\frac{π}{6}) \sin (\frac{π}{4}))$

The exact value of $\sin (\frac{π}{6})$ is $\frac{1}{2}$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{1}{2} \cos (\frac{π}{4}) + \cos (\frac{π}{6}) \sin (\frac{π}{4}))$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \cos (\frac{π}{6}) \sin (\frac{π}{4}))$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \frac{\sqrt{3}}{2} \sin (\frac{π}{4}))$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2})$

Simplify $\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2}$ .

Simplify each term.

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

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{2 \cdot 2} + \frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2})$

Multiply $2$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{4} + \frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2})$

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

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{4} + \frac{\sqrt{3} \sqrt{2}}{2 \cdot 2})$

Combine using the product rule for radicals.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{4} + \frac{\sqrt{3 \cdot 2}}{2 \cdot 2})$

Multiply $3$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{4} + \frac{\sqrt{6}}{2 \cdot 2})$

Multiply $2$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} (\frac{\sqrt{2}}{4} + \frac{\sqrt{6}}{4})$

Combine the numerators over the common denominator.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2}}{2} \cdot \frac{\sqrt{2} + \sqrt{6}}{4}$

Multiply $- \frac{\sqrt{2}}{2} \cdot \frac{\sqrt{2} + \sqrt{6}}{4}$ .

Multiply $\frac{\sqrt{2} + \sqrt{6}}{4}$ by $\frac{\sqrt{2}}{2}$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{(\sqrt{2} + \sqrt{6}) \sqrt{2}}{4 \cdot 2}$

Multiply $4$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{(\sqrt{2} + \sqrt{6}) \sqrt{2}}{8}$

Apply the distributive property.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2} \sqrt{2} + \sqrt{6} \sqrt{2}}{8}$

Combine using the product rule for radicals.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2 \cdot 2} + \sqrt{6} \sqrt{2}}{8}$

Combine using the product rule for radicals.

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2 \cdot 2} + \sqrt{6 \cdot 2}}{8}$

Simplify each term.

Multiply $2$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{4} + \sqrt{6 \cdot 2}}{8}$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{\sqrt{2^{2}} + \sqrt{6 \cdot 2}}{8}$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{2 + \sqrt{6 \cdot 2}}{8}$

Multiply $6$ by $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 + \sqrt{12}}{8}$

Rewrite $12$ as $2^{2} \cdot 3$ .

Factor $4$ out of $12$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 + \sqrt{4 (3)}}{8}$

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

$- \frac{\sqrt{3} - 1}{4} - \frac{2 + \sqrt{2^{2} \cdot 3}}{8}$

Pull terms out from under the radical.

$- \frac{\sqrt{3} - 1}{4} - \frac{2 + 2 \sqrt{3}}{8}$

Cancel the common factor of $2 + 2 \sqrt{3}$ and $8$ .

Factor $2$ out of $2$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 \cdot 1 + 2 \sqrt{3}}{8}$

Factor $2$ out of $2 \sqrt{3}$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 \cdot 1 + 2 (\sqrt{3})}{8}$

Factor $2$ out of $2 \cdot 1 + 2 (\sqrt{3})$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 \cdot (1 + \sqrt{3})}{8}$

Cancel the common factors.

Factor $2$ out of $8$ .

$- \frac{\sqrt{3} - 1}{4} - \frac{2 \cdot (1 + \sqrt{3})}{2 (4)}$

Cancel the common factor.

$- \frac{\sqrt{3} - 1}{4} - \frac{2 \cdot (1 + \sqrt{3})}{2 \cdot 4}$

Rewrite the expression.

$- \frac{\sqrt{3} - 1}{4} - \frac{1 + \sqrt{3}}{4}$

Step 2

Combine the numerators over the common denominator.

$\frac{- (\sqrt{3} - 1) - (1 + \sqrt{3})}{4}$

Step 3

Simplify each term.

Apply the distributive property.

$\frac{- \sqrt{3} - - 1 - (1 + \sqrt{3})}{4}$

Multiply $- 1$ by $- 1$ .

$\frac{- \sqrt{3} + 1 - (1 + \sqrt{3})}{4}$

Apply the distributive property.

$\frac{- \sqrt{3} + 1 - 1 \cdot 1 - \sqrt{3}}{4}$

Multiply $- 1$ by $1$ .

$\frac{- \sqrt{3} + 1 - 1 - \sqrt{3}}{4}$

Step 4

Simplify terms.

Subtract $\sqrt{3}$ from $- \sqrt{3}$ .

$\frac{1 - 1 - 2 \sqrt{3}}{4}$

Simplify by subtracting numbers.

Subtract $1$ from $1$ .

$\frac{0 - 2 \sqrt{3}}{4}$

Subtract $2 \sqrt{3}$ from $0$ .

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

Cancel the common factor of $- 2$ and $4$ .

Factor $2$ out of $- 2 \sqrt{3}$ .

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

Cancel the common factors.

Factor $2$ out of $4$ .

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

Cancel the common factor.

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

Rewrite the expression.

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

Move the negative in front of the fraction.

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

Step 5

The result can be shown in multiple forms.

Exact Form:

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

Decimal Form:

$- 0.86602540 \dots$

Find the Exact Value cos((3pi)/4)cos((5pi)/12)-sin((3pi)/4)sin((5pi)/12)

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