Precalculus Examples

$225 °$

Step 1

Evaluate $\cos (225 °)$ .

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Step 1.1

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.

$- \cos (45)$

Step 1.2

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

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

Step 2

Set up the coordinates $(\cos (θ), \sin (θ))$ .

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