Why is cos (-θ) = cos θ?

[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.

 

[Dr.Peterson]

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.

 

[stapel]

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)

 

[Dr.Peterson]

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.

 

[Indranil]

I understand it. Thank you all.