Trigonometry Identities

all science teachers crazy maths tuition

Negative angles:

  • \sin (-x)=-\sin x
  • \boxed{\cos (-x)=\cos x}        (Still Positive!)
  • \tan (-x)=-\tan x

Reason: -x is in the “C” quadrant so Cosine is still positive.

Supplementary angles:

  • \boxed{\sin (180^\circ -x)=\sin x}        (Still positive!)
  • \cos (180^\circ -x)=-\cos x
  • \tan (180^\circ -x)=-\tan x

Reason: 180^\circ -x is in the “S” quadrant so Sine is still positive.

Complementary angles:

  • \sin (90^\circ -x)=\cos x
  • \cos (90^\circ -x)=\sin x
  • \tan (90^\circ -x)=\cot x

Reason: right angled triangle 90-x

  • \sin (90^\circ -x)=\cos x=a/c
  • \cos (90^\circ -x)=\sin x=b/c
  • \tan (90^\circ -x)=\cot x=a/b

Author: mathtuition88

Math and Education Blog

One thought on “sin(180-x)”

  1. Reblogged this on Singapore Maths Tuition and commented:

    I realize that many students from top IP schools don’t know these formula! (Not entirely their fault since the teachers don’t teach/emphasize it.)
    Nevertheless, they are often tested in the harder A Math Trigonometry questions, it is a must to know if you are aiming for A1.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: