Why is cos (-θ) = cos θ?

I

Indranil

Junior Member
Joined
Feb 22, 2018
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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.
 
Indranil said:
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.
Click to expand...

What definition have you been given for the cosine (of a non-acute angle)? We can start there.
 
Indranil said:
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.
Click to expand...
Think about what you learned back in algebra about even and odd functions.... (here) ;)
 
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