# Thread: Why is cos (-θ) = cos θ?

1. ## Why is cos (-θ) = cos θ?

when the angle (say) theta is negative,

1) sin (-θ) = - sin θ

2. tan (-θ) = - tan -θ

then why,

3. cos (-θ) = cos θ? Could you explain this point here, please? I am confused.

2. Originally Posted by Indranil
when the angle (say) theta is negative,

1) sin (-θ) = - sin θ

2. tan (-θ) = - tan -θ

then why,

3. cos (-θ) = cos θ? Could you explain this point here, please? I am confused.
What definition have you been given for the cosine (of a non-acute angle)? We can start there.

3. Originally Posted by Indranil
when the angle (say) theta is negative,

1) sin (-θ) = - sin θ

2. tan (-θ) = - tan -θ

then why,

3. cos (-θ) = cos θ? Could you explain this point here, please? I am confused.
Think about what you learned back in algebra about even and odd functions.... (here)

4. Originally Posted by Indranil
3. cos (-θ) = cos θ? Could you explain this point here, please? I am confused.
For two approaches, see the unit circle here, and the graph of the cosine here.

5. I understand it. Thank you all.

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