Turn on thread page Beta
 You are Here: Home >< Maths

# How does cos(120)=-cos(60)? watch

1. Is there a general formula like sin(x) = sin(180-x)?

2. I hope this helps you!
Posted on the TSR App. Download from Apple or Google Play
3. Because the calculator says so.
4. (Original post by hp4300)
Is there a general formula like sin(x) = sin(180-x)?
The two identities which make the title statement true are:

5. As shown above, cosine is negative in the second quadrant (90-180 degrees). And ignoring plus and minus, sin/cos/tan(180 - x) = sin/cos/tan(x).

So cos(120) = -cos(60)

Similarly since sin is positive in the second quadrant you have

sin(120) = sin(60)

This can all be seen by the graph, identities or CAST.

Turn on thread page Beta

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 13, 2018
Today on TSR

### Top unis in Clearing

Tons of places at all these high-ranking unis

Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE