Find the Exact Value sin(-105 degrees )

$\sin (- 105 °)$

Step 1

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

$- \sin (75)$

Step 2

Split $75$ into two angles where the values of the six trigonometric functions are known.

$- \sin (30 + 45)$

Step 3

Apply the sum of angles identity.

$- (\sin (30) \cos (45) + \cos (30) \sin (45))$

Step 4

The exact value of $\sin (30)$ is $\frac{1}{2}$ .

$- (\frac{1}{2} \cos (45) + \cos (30) \sin (45))$

Step 5

The exact value of $\cos (45)$ is $\frac{\sqrt{2}}{2}$ .

$- (\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \cos (30) \sin (45))$

Step 6

The exact value of $\cos (30)$ is $\frac{\sqrt{3}}{2}$ .

$- (\frac{1}{2} \cdot \frac{\sqrt{2}}{2} + \frac{\sqrt{3}}{2} \sin (45))$

Step 7

The exact value of $\sin (45)$ is $\frac{\sqrt{2}}{2}$ .

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

Step 8

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 $\frac{\sqrt{2}}{2}$ .

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

Multiply $2$ by $2$ .

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

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

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

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

Combine using the product rule for radicals.

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

Multiply $3$ by $2$ .

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

Multiply $2$ by $2$ .

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

Combine the numerators over the common denominator.

$- \frac{\sqrt{2} + \sqrt{6}}{4}$

Step 9

The result can be shown in multiple forms.

Exact Form:

$- \frac{\sqrt{2} + \sqrt{6}}{4}$

Decimal Form:

$- 0.96592582 \dots$

Find the Exact Value sin(-105 degrees )

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